Hey guess what, Gamefreak did the thing: we get the full SV dex (except for Ubers and the DLC paradoxes). This leads to an overwhelming amount of Pokemon for Regulation D! So this thread will help you prepare, with showing what the new Pokemon are, and what stat quirks to take into account for them! I'll divide them between several categories and stuff obtainable in other games to help with gathering your mons!
People who have helped added stuff / fix mistakes, make sure to thank them!
Thick Fat Azumarill
Wild Eep
Rain Moon
Optimizations
Before we get started, each Pokemon will have some optimizations you can do to make them as good as they can be! I'll explain the reasoning behind each of them now to save space later!
IVs: Lower Attack IVs are the best for Foul Play damage, 0 Speed IVs are for Trick Room. Lower Special Attack IVs are to avoid Tera Blast changing to a Special Attack after getting lowered, which can be a big help when using Choiced Pokemon against Charm Flutter Mane, and attack drops from Mimikyu's Play Rough.
Gender: If a Pokemon uses contact moves, a Female Gender is preferred if possible to avoid getting hit by Enamorus's potential Cute Charm. This isn't just physical attacks: Grass Knot and Draining Kiss are notably contact as well!
Neutral Nature: If using Trick or Switcheroo, a neutral nature is preferred as the base nature before you use a nature mint, so you can eat any berry without getting confused.
Egg Moves: These are listed so if you breed a Pokemon in SV, you can just relearn these via the move relearner instead of having to use a mirror herb to get them back! I also mention Egg moves if they can save you a TM use. I'll also mention if a move is only relevant for doubles.
Abilites: I'll list any relevant Hidden Abilities.
Home Relearn Moves: Yes I know "move transfers" are dead, but this is something different: you can retain moves that are legal in the SV learnset if you transfer the Pokemon over to SV, via the Home move relearner. You can save on TM resources this way, though they won't show up in the SV move relearner! I will also mention if a move is only relevant for doubles. You'll have to find out yourself which games to get them in though, it'd take forever for me to list every one lol.
New Moves and Lost Moves: I'll mention any relevant new / lost moves, including returning moves from older gens.
Legendary Pokemon That Require Home
The Hellish Hisuian Pokemon: Enamorus 
You don't even know its IVs until you transfer it, which is awful!
You CAN RNG it though, thanks for Charlotte for bringing this up! More info can be found here: https://github.com/AskMeAboutBirds/PLA-genie-guide
Optimal Stats:
5 Attack IV or lower for minimum Foul Play damage on Offensive Sets or sets with just 252 HP EVs
3 Attack IV or lower for minimum Foul Play damage on full defensive Bold sets
1 Speed IV or lower for Trick Room for Therian forme only
Home Relearn Moves: Play Rough, Calm Mind, Sludge Bomb, Earth Power, Zen Headbutt, Psychic, Rest
The Rest of the Legendary Pokemon 
Time to cover all the other legal Legendary Pokemon for Regulation D! I'll mention what games they're obtainable in. Additionally, I'll divide that between the easiest way to get Pokemon optimally without RNGing them, and why that is! If you're RNGing, you probably know what you're doing more than me, so don't worry about this! Also I will mention especially bad games to get mons for fun lol.
One last thing: I have non 0 minimum IV stuff listed if you don't want to go all the way in optimizing stats, since most Pokemon don't entirely need a 0 to make damage calcs go their way!
Articuno
Optimal Stats: 5 Atk IV or lower for minimum Foul Play damage
Best games for getting Snow Cloak without Ability Patch: Red / Green / Blue / Yellow (can use exp number upon Poketransfer to guarantee any desired nature, though 3 31 IVs are guaranteed. Can also easily clone them before transfer via Gold / Silver)
Best games for getting Pressure: Heart Gold / Soul Silver (best chance of getting low Attack IV, 50% chance of desired nature with Synchronize)
Other games: Fire Red / Leaf Green (best chance of getting low Attack IV, Synchronize does NOT work)
Let's Go Pikachu / Let's Go Eevee (guaranteed desired nature if using Fortune Teller in Celedon City Pokemon Center, can get multiple via rare flying ride spawn)
Shining Pearl (Synchronize works 100% of the time)
X / Y (requires having chosen Chespin)
Ultra Sun / Ultra Moon
Awful Games: XD (can't even check its stats till after the credits!)
Platinum (it roams!)
Sword / Shield (can't check stats until after Dynamax Adventure)
Pokemon Go (low Attack IV is impossible without Pokemon Go trade, nature is locked in upon transfer)
Potentially Relevant Home Relearn Moves: Icy Wind, Light Screen, Protect, Substitute, U-turn
New Moves: Helping Hand, Snowscape, Tera Blast
Lost Moves: Weather Ball
Galarian Articuno
Optimal Stats: 3 Atk IV or lower for minimum Foul Play damage
Other Optimizations: Neutral Nature if using Trick
The only good game to obtain it in: Sword / Shield DLC
The worst game to obtain it in: Pokemon Go (can't get low Attack IV without a trade, nature is locked in upon transfer)
Potentially Relevant Home Relearn Moves: Calm Mind, Imprison, Light Screen, Protect, Shadow Ball, Stored Power, Substitute, U-turn
New Moves: Helping Hand, Snowscape, Tera Blast
Azelf
Optimal Stats
5 Atk IV or lower for minimum Foul Play damage (assuming an offensive set)
0 in Atk or SpA to retain the same physical / special category on Tera Blast after 1 stat drop
Other Optimizations: Neutral Nature if using Trick
Best games: Diamond / Pearl / Platinum / Black 2 / White 2 (Best chance to get low Attack IV, 50% chance of Synchronize working for nature), Brilliant Diamond / Shining Pearl (Synchronize always works for desired nature)
Other games: Omega Ruby / Alpha Sapphire / Ultra Sun / Ultra Moon
Awful Games: Pokemon Go (can't get low Attack IVs without trading, nature is locked in upon transfer)
Sword / Shield (can't check stats until after Dynamax Adventure)
Legends (can't check IVs until transfer)
Potentially Relevant Home Relearn Moves: Baton Pass, Dazzling Gleam, Encore, Fire Blast, Fire Punch, Flamethrower, Grass Knot, Ice Punch, Light Screen, Play Rough, Psyshock, Reflect, Stealth Rock, Taunt, Thunder Punch, Thunder Wave, Thunderbolt, Trick, U-turn, Zen Headbutt
New Moves: Mystical Power, Tera Blast
Cresselia
Optimal Stats: 3 Atk IV or lower for minimum Foul Play damage
3 Spe IV or lower for Trick Room only
Other Optimizations: Neutral Nature if using Trick
Best games: Black 2 / White 2 (doesn't roam, best chance for low Attack IV, Synchronize works 50% of the time)
Other games: Omega Ruby / Alpha Sapphire / Ultra Sun / Ultra Moon
Awful Games: Diamond / Pearl / Platinum / Shining Pearl / Brilliant Diamond (it roams!)
Sword / Shield (can't check stats until after Dynamax Adventure)
Legends (can't check IVs until transfer)
Pokemon Go (can't get low Attack IV without a trade, nature is locked in upon transfer)
Potentially Relevant Home Relearn Moves: Calm Mind, Charge Beam, Grass Knot, Helping Hand (doubles only), Ice Beam, Icy Wind, Light Screen, Protect, Psyshock, Reflect, Rest, Sleep Talk, Stored Power, Substitute, Thunder Wave, Thunderbolt, Trick, Trick Room
New Moves: Lunar Blessing, Tera Blast
Glastrier
Optimal Stats: 1 Spe IV or lower for Trick Room only.
The Only Game: Sword / Shield (DLC Required)
Potentially Relevant Home Relearn Moves: Close Combat, Heavy Slam, Protect, Rest, Sleep Talk, Stomping Tantrum, Substitute, Zen Headbutt
New Moves: Snowscape, Tera Blast, Trailblaze
Heatran
Optimal Stats:
1 Attack IV or lower for minimum Foul Play Damage
1 Spe or lower (Trick Room only)
Other Optimizations: Female Gender if using Heavy Slam or Flame Charge
Best Games: Diamond / Pearl / Platinum / Black 2 / White 2 (best chance to get low Attack IV, 50% Synchronize chance)
Other Games: Omega Ruby / Alpha Sapphire / Ultra Sun
Awful Games: Sword / Shield (can't check stats until after Dynamax Adventure)
Legends (can't check IVs until transfer)
Pokemon Go (can't get low Attack IV without a trade, nature is locked in upon transfer)
Potentially Relevant Home Relearn Moves: Body Press, Bulldoze, Fire Blast, Flame Charge, Flamethrower, Flash Cannon, Heavy Slam, Iron Defense, Overheat, Protect, Rest, Rock Tomb, Sleep Talk, Solar Beam, Stealth Rock, Steel Beam, Substitute, Sunny Day, Taunt, Will-O-Wisp
New Moves: Pounce, Power Gem, Tera Blast
Landorus / Landorus-T
Optimal Stats:
1 Special Attack IV or lower for keeping Tera Blast physical on Landorus-T with the minimum Attack EV investment after -2 Drop (124 EVs with Adamant)
19 Special Attack IV or lower for keeping Tera Blast physical on Adamant Landorus-T with maximum Attack EV investment after -2 Drop
5 Attack IV or lower for minumum Foul Play Damage (if using Sheer Force Landorus with only Special Attacks)
Best Games For Landorus-T (don't require both Thundurus and Tornadus, but do not give you easy control over nature / IVs):
Sword / Shield / Legends / Pokemon Go / Pokemon Dream Radar + Black 2 or White 2
Best Games for regular Landorus (all require to have Thundurus and Tornadus in your party, but offer more control over IVs / Nature)
Black / White / Omega Ruby / Alpha Sapphire / Ultra Sun / Ultra Moon
Potentially Relevant Home Relearn Moves (Landorus-T): Bulk Up, Fling, Grass Knot, Imprision, Protect, Rest, Scary Face, Sleep Talk, Stealth Rock, Stomping Tantrum (Doubles Only), Stone Edge, Substitute, Swords Dance, U-turn
Potentially Relevant Home Relearn Moves (Landorus): Calm Mind, Focus Blast, Imprison, Psychic, Rain Dance, Sludge Bomb, Stealth Rock, Stone Edge, Substitute, U-turn
New Moves: Nasty Plot, Rain Dance, Sandsear Storm, Smack Down, Sunny Day, Taunt, Tera Blast
Lost Moves: Superpower
Mesprit
Optimal Stats
3 Attack IV or lower for minimum Foul Play damage
11 Offensive IV or lower for Tera Blast to stay physical or special with the maximum offensive investment after -1 Drop
1 Speed IV or lower for Trick Room only
Other Optimizations: Neutral Nature if using Trick
Best games: Black 2 / White 2 (Best chance to get low Attack IV, 50% chance of Synchronize working for nature)
Other games: Omega Ruby / Alpha Sapphire / Ultra Sun / Ultra Moon
Awful Games:
Diamond / Pearl / Platinum / Brilliant Diamond / Shining Pearl (It roams!)
Pokemon Go (can't get low Attack IVs without trading, nature is locked in upon transfer)
Sword / Shield (can't check stats until after Dynamax Adventure)
Legends (can't check IVs until transfer)
Potentially Relevant Home Relearn Moves: Baton Pass, Charm, Dazzling Gleam, Draining Kiss, Encore, Fire Punch, Fling, Grass Knot, Ice Beam, Ice Punch, Light Screen, Nasty Plot, Play Rough, Psyshock, Reflect, Stealth Rock, Substitute, Thunderbolt, Thunder Punch, Thunder Wave, Trick, U-turn, Zen Headbutt
New Moves: Mystical Power, Tera Blast
Moltres
Optimal Stats: 1 Atk IV or lower for minimum Foul Play damage
Best games for getting Flame Body without Ability Patch: Red / Green / Blue / Yellow (can use exp number upon Poketransfer to guarantee any desired nature, though 3 31 IVs are guaranteed. Can also easily clone them before transfer via Gold / Silver)
Best games for getting Pressure: Heart Gold / Soul Silver (best chance of getting low Attack IV, 50% chance of desired nature with Synchronize)
Other games:
Fire Red / Leaf Green (best chance of getting low Attack IV, Synchronize does NOT work)
Let's Go Pikachu / Let's Go Eevee (guaranteed desired nature if using Fortune Teller in Celedon City Pokemon Center, can get multiple via rare flying ride spawn)
Shining Pearl (Synchronize works 100% of the time)
X / Y (requires having chosen Chespin)
Ultra Sun / Ultra Moon
Awful Games:
XD (can't even check its stats till after the credits!)
Platinum (it roams!)
Sword / Shield (can't check stats until after Dynamax Adventure)
Go (low Attack IV is impossible without Pokemon Go trade, nature is locked in upon transfer)
Potentially Relevant Home Relearn Moves: Fire Blast, Flame Charge, Flamethrower, Solar Beam, Substitute, Tailwind, U-turn
New Moves: Helping Hand, Tera Blast
Lost Moves: Scorching Sands, Weather Ball
Galarian Moltres
Optimal Stats: 3 Atk IV or lower for minimum Foul Play damage
The only good game to obtain it in: Pokemon Sword / Shield DLC
The worst game to obtain it in: Pokemon Go (can't get low Attack IV without a trade, nature is locked in upon transfer)
Potentially Relevant Home Relearn Moves: Foul Play, Rest, Sleep Talk, Snarl, Substitute, Tailwind, Taunt, U-turn
New Moves: Helping Hand, Tera Blast
Regidrago
Optimal Stats:
3 Attack IV or lower for minimum Foul Play damage (assuming offensive EV spread), 1 Atk IV or lower for all spreads
15 Offensive IV or lower for Jolly / Timid Regidrago to retain Special / Physical on Tera Blast with maximum offensive investment after -1 Drop
The only good game to obtain it in: Sword / Shield DLC
The worst game to obtain it in: Pokemon Go (can't get low Attack IV without a trade, nature is locked in upon transfer)
Potentially Relevant Home Relearn Moves: Crunch, Draco Meteor, Dragon Dance, Fire Fang, Outrage, Substitute
New Moves: Earthquake, Earth Power, Ice Fang, Tera Blast
Regieleki
Optimal Stats
1 Attack IV or lower for minimum Foul Play damage on all sets
3 Attack IV or lower for minimum Foul Play damage on full offensive sets
15 Offensive IV or lower for Jolly / Timid Regieleki to retain Special / Physical on Tera Blast with maximum offensive investment after -1 Drop
The only good game to obtain it in: Pokemon Sword / Shield DLC
The worst game to obtain it in: Pokemon Go (can't get low Attack IV without a trade, nature is locked in upon transfer)
Potentially Relevant Home Relearn Moves: Acrobatics, Light Screen, Reflect, Substitute, Thunder, Volt Switch
New Moves: Charge Beam, Tera Blast
Lost Moves: Assurance
Spectrier
Optimal Stats: 3 Attack IV or lower for minimum Foul Play damage
The Only Game: Sword / Shield (DLC Required)
Potentially Relevant Home Relearn Moves: Calm Mind, Dark Pulse, Foul Play, Mud Shot, Taunt, Snarl, Substitute, Will-O-Wisp
New Moves: Draining Kiss, Night Shade, Psychic, Tera Blast
Thundurus / Thundurus-T
Optimal Stats:
5 Attack IV or lower for minimum Foul Play damage (for Regular Thundurus)
3 Attack IV or lower for minimum Foul Play damage (for Thundurus-T)
Best Games:
White (Roaming, but best chance of gettting low Attack IV)
Dream Radar + Black 2 / White 2 (Can't soft reset for IVs, but Defiant is guaranteed)
Other Games: Alpha Sapphire / Ultra Moon
Awful Games:
Shield (can't check stats until after Dynamax Adventure)
Legends (can't check stats until after transfer)
Pokemon Go (can't get low Attack IV without a trade, nature is locked in upon transfer)
Potentially Relevant Home Relearn Moves: Brick Break, Bulk Up, Charge Beam, Crunch, Grass Knot, Eerie Impulse, Fling, Focus Blast, Foul Play, Nasty Plot, Scary Face, Sludge Bomb, Substitute, Taunt, Thunderbolt, Thunder Wave, U-turn, Wild Charge
New Moves: Acrobatics, Charge Beam, Snarl, Sunny Day, Tera Blast, Wildbolt Storm
Lost Moves: Electroweb, Superpower
Tornadus / Tornadus-T
Optimal Stats:
5 Attack IV or lower for minimum Foul Play damage (for Regular Tornadus)
3 Attack IV or lower for minimum Foul Play damage (for Tornadus-T)
Best Games:
Black (Roaming, but best chance of gettting low Attack IV)
Dream Radar + Black 2 / White 2 (Can't soft reset for IVs, but Defiant is guaranteed)
Other Games: Omega Ruby / Ultra Sun
Awful Games:
Sword (can't check stats until after Dynamax Adventure)
Legends (can't check stats until after transfer)
Pokemon Go (can't get low Attack IV without a trade, nature is locked in upon transfer)
Potentially Relevant Home Relearn Moves: Acrobatics, Brick Break, Bulk Up, Crunch, Grass Knot, Fling, Focus Blast, Foul Play, Heat Wave, Icy Wind, Nasty Plot, Scary Face, Sludge Bomb, Substitute, Taunt, U-turn
New Moves: Snowscape, Sunny Day, Tera Blast, Bleakwind Storm
Lost Moves: Superpower
Urshifu-R
Optimizations: Female Gender
The Only Game: Sword / Shield (DLC Required, but can evolve in SV from Kubfu via Scroll of Waters from Auction House)
Potentially Relevant Home Relearn Moves: Bulk Up, Drain Punch, Fire Punch, Iron Head, Poiosn Jab, Rock Slide, Rock Tomb, Stone Edge, Substitute, Taunt, Thunder Punch, U-turn
New Moves: Fling, Ice Spinner, Swords Dance, Tera Blast, Trailblaze
Urshifu-S
Optimizations: Female Gender
The Only Game: Sword / Shield (DLC Required, but can evolve in SV from Kubfu via Scroll of Darkness from Auction House)
Potentially Relevant Home Relearn Moves: Bulk Up, Drain Punch, Fire Punch, Ice Punch, Iron Head, Poiosn Jab, Rock Slide, Rock Tomb, Stone Edge, Substitute, Taunt, Thunder Punch, U-turn
New Moves: Swords Dance, Tera Blast, Trailblaze
Uxie
Optimal Stats
5 Attack IV or lower for minimum Foul Play damage if using Max HP / Max Defense Bold
3 Attack IV or lower for minimal Foul Play damage on all sets
1 Speed IV or lower for Trick Room Only
Other Optimizations: Neutral Nature if using Trick
Best games:
Diamond / Pearl / Platinum / Black 2 / White 2 (Best chance to get low Attack IV, 50% chance of Synchronize working for nature), Brilliant Diamond / Shining Pearl (Synchronize always works for desired nature)
Other games: Omega Ruby / Alpha Sapphire / Ultra Sun / Ultra Moon
Awful Games:
Pokemon Go (can't get low Attack IVs without trading, nature is locked in upon transfer)
Sword / Shield (can't check stats until after Dynamax Adventure)
Legends (can't check IVs until transfer)
Potentially Relevant Home Relearn Moves: Baton Pass, Calm Mind, Dazzling Gleam, Draining Kiss, Encore, Fire Blast, Fire Punch, Flamethrower, Fling, Grass Knot, Ice Punch, Light Screen, Nasty Plot, Play Rough, Psyshock, Reflect, Rest, Sleep Talk, Stealth Rock, Stored Power, Thunder Punch, Thunder Wave, Thunderbolt, Trick, Trick Room U-turn, Zen Headbutt
New Moves: Mystical Power, Tera Blast
Zapdos
Optimal Stats: 1 Attack IV or lower for minimum Foul Play damage
Best games for getting Static without Ability Patch: Red / Green / Blue / Yellow (can use exp number upon Poketransfer to guarantee any desired nature, though 3 31 IVs are guaranteed. Can also easily clone them before transfer via Gold / Silver)
Best games for getting Pressure: Heart Gold / Soul Silver (best chance of getting low Attack IV, 50% chance of desired nature with Synchronize)
Other games:
Fire Red / Leaf Green (best chance of getting low Attack IV, Synchronize does NOT work)
Let's Go Pikachu / Let's Go Eevee (guaranteed desired nature if using Fortune Teller in Celedon City Pokemon Center, can get multiple via rare flying ride spawn)
Shining Pearl (Synchronize works 100% of the time)
X / Y (requires having chosen Chespin)
Ultra Sun / Ultra Moon
Awful Games:
XD (can't even check its stats till after the credits!)
Platinum (it roams!)
Sword / Shield (can't check stats until after Dynamax Adventure)
Pokemon Go (low Attack IV is impossible without Pokemon Go trade, nature is locked in upon transfer)
Potentially Relevant Home Relearn Moves: Baton Pass, Eerie Impulse, Heat Wave, Hurricane, Tailwind, Thunderbolt, Substitute, U-turn, Volt Switch
New Moves: Air Cutter, Helping Hand, Tera Blast
Lost Moves: Weather Ball
Galarian Zapdos
Obtainable Games: Sword / Shield (DLC only), Pokemon Go (rare Daily Incense spawn)
Potentially Relevant Home Relearn Moves: Brave Bird, Tailwind, Taunt, Substitute, U-turn
Lost Moves: Blaze Kick, Throat Chop
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Pixie Plate: transfer Arceus to your game, get it from the auction area. It could be useful for some Fairy Pokemon to get around Item Clause, it still gives a 20% boost to Fairy-type Attacks!
Flutter Mane: 
Sylveon 
Heal Bell on Blissey / Chansey (learn Heal Bell in a different game such as SwSh, relearn upon transfer in the Home menu for SV)
Cosmic Power Rabsca
Simple Beam Golduck and Grumpig
Barb Barrage Qwilfish
Psyshield Bash Stantler
Raging Fury Arcanine