One thing lots of people overlook here are that A) Ubers are still a thing, and B) that weather still exists. So im here to go ahead and forget everything i learned while playing this OM and make probably 50 errors yes
Sun
Setters
Groudon - 100/150/140/100/90/90; Practically the best Pokemon with its Primal, so most people will expect to see it rather than standard. Hits like a truck, takes hits like a tank, and has probably the best movepool to do what it wants to do.
Vulpix - 76/82/80/100/130/130; Can carry Pain Split for semi-reliable recovery and set up burns much more easily than Lava Plume via Will-O-Wisp, as well as being a faster Sun setter who is more specially oriented. Struggles to beat Kyogre, Pogre, Kyurems, hell, doesn't really have any offensive presence besides maybe hitting Stufful pretty hard with Fire Blast.
Abusers Phys
Deerling- The only real one besides something like Ho-Oh, which just likes Water damage reduced and bonus Sacred Fire damage, that really benefits and is physical. Chlorophyll plus dual STAB that works against nearly every type, and Jump Kick to work against those that those two can't hit means you hit everything pretty damn hard. You also outspeed Xerneas in Sun after Geomancy and can speedtie with Deoxys Normal and Attack.
Abusers Spec
Nuzleaf - Odd, but workable movepool catered towards mixed offenses, but both of its best STAB moves are Special, so I'll place him here (less useful overall than Lombre for Rain, see ahead). Nature Power can help in terrains, while Extrasensory hits several Poisons like Koffing and Toxapex. Rock Slide can help with Ho-Oh, which otherwise hard walls Nuzleaf. Also, Growth can get a bit busted if you can keep Sun up.
Charmander - Spoken of in the base post of this thread, Z-Sunny Day with Solar Power's a cool niche while hitting like a goddamn truck using magic. Just don't expect to kill Ho-Oh without Hidden Power Rock, and get walled by Heatran without Hidden Power Ground.
Helioptile - More niche pick that can also be a tool to beat Rain via Dry Skin. A limited movepool with Thunderbolt, Hyper Voice, Surf, Dark Pulse, and Grass Knot being your best bets past Hidden Power, but also can run a bit of a support set with Thunder Wave or Glare and Volt Switch or U-Turn for pivoting.
Rain
Setters
Kyogre - 100/100/90/150/140/90; Similarly to Pdon most people may expect a Primal Kyogre, but regular Kyogre is also fairly common. Calm Minds tons or can run Scarf Water Spot, or RestTalk for a more defensive set to keep setting rain into a match.
Pelliper - 60/50/100/95/70/65; Can carry recovery and pivoting through U-Turn, as well as a nice immunity to Pdon's main STAB in Precipice Blades or Earthquake. Past that, it's basically fully outclassed.
Abusing Phys
Buizel - 110/130/70/120/60/170; Can struggle due to a lack of Swords Dance or attacking moves besides Pursuit, STAB options, and Ice Punch, but can Pursuit trap a few mons, possibly go mixed with Ice Beam, can Switcheroo items away from things like Xerneas (but shits itself once it realizes that Xern is Fairium).
Kabutops - 60/115/105/65/70/80; Can trouble Pdon with Z-Stone Edge and threaten most physical walls or fat in the physical department ones like Koffing, Pawniard, or Scraggy by hitting hard in the rain with Waterfall and/or setting up on fat things with little offensive presence with Swords Dance.
Abusing God
Lombre - 120/100/100/120/140/100; A varied movepool including oddities like all three of the Elemental Punches, Knock Off, Ice Beam, and STAB attacks for both Physical and Special sets, Lombre's cool. Nature Power works for running alongside something like Thunder Tapu Koko to give a more specially inclined Lombre a bit better coverage, as well as being able to take on Primal Groudon fairly well since it isnt weak to Stone Edge, yet still resists Precipice Blades, takes neutral damage from Lava Plume with great 120/140 Special bulk, and has a neutral healing move to damage Pdon while chipping at it (or while protectstalling with Leech Seed).
Respekt Lombre.
Abusers Special
Psyduck, Finneon, Mantyke - Variable; They all fill pretty similar roles; Psyduck hits harder than the others and is probably the best raw offensive one, Finneon carries Defog but the offensive stats are pathetic alongside a bad movepool, and Mantyke is a better defensive wall but is capable of taking Pdon on outside of Rain due to a resistance to Lava Plume and not taking damage from Precipice. If you don't want to use Lombre, the best bet for a real offensive one is Psyduck with access to Calm Mind and Psychic for a bit more presence.
Sand
Setters
Tyranitar - 100/134/110/95/100/61; Still as good as ever. Struggles to take too many hits due to a bad defensive typing and no recovery, but has a much stronger offensive presence than Hippopotas, can Pursuit Trap the likes of Aegislash, Frillish, and non-HP Fighting Abra and Gastly.
Tyranitar-Mega - 100/164/150/95/120/71; Takes up your Mega slot, can't effectively beat other Weather setters bar its weaker non-mega'd self (who can still run Scarf Superpower or just Smooth Rock to outclass it). Not great, but its stats definitely can compete, but it struggles with taking Fighting hits.
Hippopotas - 108/112/118/78/82/72; Essentially a slightly more specially fat and faster Hippowdon, can run Slack Off for recovery, which is the only reason it's capable of competing with Tyranitar besides a typing that doesn't leave it weak to Xerneas's Moonblast and Solgaleo's Sunsteel Strike.
Abusers (they're all physical...)
Drilbur - 120/170/80/60/90/136; Faster and higher damage output than Excadrill, but lacks a bit of the coverage from Iron Head and loses many resistances also from the lack of Steel typing. Also functions as a spinner.
Excadrill - 110/135/60/50/65/88; Can help check things such as Primal Groudon, Clefairy, Spritzee, and Xerneas lacking Focus Miss sheerly through Steel typing and good Speed, but is noticeably slower and weaker than Drilbur. Also can function as a Spinner, like its devolved counterpart.
Sandshrew - 100/150/170/40/60/80; Functions better as a sand-abusing Spinner than Drilbur or Excadrill, but lacks the damage that they'll do either through lower Attack and Speed than Drilbur, or the lack of good extra typing and STAB Iron Head like Excadrill has.
Hail
Setters
Snover - 120/124/100/124/120/80; harder-hitting mixed Weather setter. Too bad it basically dies to nearly every Ubers mon that's good, dying to PDon, Dialga, Xerneas with Geomancy, Mewtwo with both forms, and a long list of 350 cup-buffed mons like Pawniard, Mienfoo, Scraggy, Stufful, et cetera.
Vulpix-Alola - 76/82/80/100/130/130; Faster and can carry Aurora Veil. Still dies to a bunch of things, notably Steels including Solgaleo.
Abusers
Kyurem-White - 125/120/90/170/100/95; Hits like a truck with Blizzard, and you already know how hard it hits since it doesn't change in 350 cup! Can struggle with breaking a few of the superbulky walls like Mantyke, but they struggle to do anything back to you, so just keep clicking Blizzard until they send in one of the many base 100 or faster speed Steels like Pawniard.
Sandshrew-Alola - 100/150/180/20/70/80; Can double up for Aurora Veil, Swords Dance is pretty cool, Icicle Crash + Iron Head is cool, and can carry Rock Slide + Earthquake for great coverage thanks to being a Sandshrew still. Also can function as a Spinner, decent defensive typing thanks to Steel typing.
Cubchoo - 110/140/80/120/80/80; Only niche over Sandshrew is the possibility of running a Mixed set and a different possible movepool with Superpower, Play Rough, and Low Kick. Also doesn't have Icicle Crash, sadly :[
If I made any errors just remind me of how bad I am at making Smogon posts, its been a little while since I've played any serious 350 Cup games with anyone with a brain.
Sun
Setters
Groudon - 100/150/140/100/90/90; Practically the best Pokemon with its Primal, so most people will expect to see it rather than standard. Hits like a truck, takes hits like a tank, and has probably the best movepool to do what it wants to do.
Vulpix - 76/82/80/100/130/130; Can carry Pain Split for semi-reliable recovery and set up burns much more easily than Lava Plume via Will-O-Wisp, as well as being a faster Sun setter who is more specially oriented. Struggles to beat Kyogre, Pogre, Kyurems, hell, doesn't really have any offensive presence besides maybe hitting Stufful pretty hard with Fire Blast.
Abusers Phys
Deerling- The only real one besides something like Ho-Oh, which just likes Water damage reduced and bonus Sacred Fire damage, that really benefits and is physical. Chlorophyll plus dual STAB that works against nearly every type, and Jump Kick to work against those that those two can't hit means you hit everything pretty damn hard. You also outspeed Xerneas in Sun after Geomancy and can speedtie with Deoxys Normal and Attack.
Abusers Spec
Nuzleaf - Odd, but workable movepool catered towards mixed offenses, but both of its best STAB moves are Special, so I'll place him here (less useful overall than Lombre for Rain, see ahead). Nature Power can help in terrains, while Extrasensory hits several Poisons like Koffing and Toxapex. Rock Slide can help with Ho-Oh, which otherwise hard walls Nuzleaf. Also, Growth can get a bit busted if you can keep Sun up.
Charmander - Spoken of in the base post of this thread, Z-Sunny Day with Solar Power's a cool niche while hitting like a goddamn truck using magic. Just don't expect to kill Ho-Oh without Hidden Power Rock, and get walled by Heatran without Hidden Power Ground.
Helioptile - More niche pick that can also be a tool to beat Rain via Dry Skin. A limited movepool with Thunderbolt, Hyper Voice, Surf, Dark Pulse, and Grass Knot being your best bets past Hidden Power, but also can run a bit of a support set with Thunder Wave or Glare and Volt Switch or U-Turn for pivoting.
Rain
Setters
Kyogre - 100/100/90/150/140/90; Similarly to Pdon most people may expect a Primal Kyogre, but regular Kyogre is also fairly common. Calm Minds tons or can run Scarf Water Spot, or RestTalk for a more defensive set to keep setting rain into a match.
Pelliper - 60/50/100/95/70/65; Can carry recovery and pivoting through U-Turn, as well as a nice immunity to Pdon's main STAB in Precipice Blades or Earthquake. Past that, it's basically fully outclassed.
Abusing Phys
Buizel - 110/130/70/120/60/170; Can struggle due to a lack of Swords Dance or attacking moves besides Pursuit, STAB options, and Ice Punch, but can Pursuit trap a few mons, possibly go mixed with Ice Beam, can Switcheroo items away from things like Xerneas (but shits itself once it realizes that Xern is Fairium).
Kabutops - 60/115/105/65/70/80; Can trouble Pdon with Z-Stone Edge and threaten most physical walls or fat in the physical department ones like Koffing, Pawniard, or Scraggy by hitting hard in the rain with Waterfall and/or setting up on fat things with little offensive presence with Swords Dance.
Abusing God
Lombre - 120/100/100/120/140/100; A varied movepool including oddities like all three of the Elemental Punches, Knock Off, Ice Beam, and STAB attacks for both Physical and Special sets, Lombre's cool. Nature Power works for running alongside something like Thunder Tapu Koko to give a more specially inclined Lombre a bit better coverage, as well as being able to take on Primal Groudon fairly well since it isnt weak to Stone Edge, yet still resists Precipice Blades, takes neutral damage from Lava Plume with great 120/140 Special bulk, and has a neutral healing move to damage Pdon while chipping at it (or while protectstalling with Leech Seed).
Respekt Lombre.
Abusers Special
Psyduck, Finneon, Mantyke - Variable; They all fill pretty similar roles; Psyduck hits harder than the others and is probably the best raw offensive one, Finneon carries Defog but the offensive stats are pathetic alongside a bad movepool, and Mantyke is a better defensive wall but is capable of taking Pdon on outside of Rain due to a resistance to Lava Plume and not taking damage from Precipice. If you don't want to use Lombre, the best bet for a real offensive one is Psyduck with access to Calm Mind and Psychic for a bit more presence.
Sand
Setters
Tyranitar - 100/134/110/95/100/61; Still as good as ever. Struggles to take too many hits due to a bad defensive typing and no recovery, but has a much stronger offensive presence than Hippopotas, can Pursuit Trap the likes of Aegislash, Frillish, and non-HP Fighting Abra and Gastly.
Tyranitar-Mega - 100/164/150/95/120/71; Takes up your Mega slot, can't effectively beat other Weather setters bar its weaker non-mega'd self (who can still run Scarf Superpower or just Smooth Rock to outclass it). Not great, but its stats definitely can compete, but it struggles with taking Fighting hits.
Hippopotas - 108/112/118/78/82/72; Essentially a slightly more specially fat and faster Hippowdon, can run Slack Off for recovery, which is the only reason it's capable of competing with Tyranitar besides a typing that doesn't leave it weak to Xerneas's Moonblast and Solgaleo's Sunsteel Strike.
Abusers (they're all physical...)
Drilbur - 120/170/80/60/90/136; Faster and higher damage output than Excadrill, but lacks a bit of the coverage from Iron Head and loses many resistances also from the lack of Steel typing. Also functions as a spinner.
Excadrill - 110/135/60/50/65/88; Can help check things such as Primal Groudon, Clefairy, Spritzee, and Xerneas lacking Focus Miss sheerly through Steel typing and good Speed, but is noticeably slower and weaker than Drilbur. Also can function as a Spinner, like its devolved counterpart.
Sandshrew - 100/150/170/40/60/80; Functions better as a sand-abusing Spinner than Drilbur or Excadrill, but lacks the damage that they'll do either through lower Attack and Speed than Drilbur, or the lack of good extra typing and STAB Iron Head like Excadrill has.
Hail
Setters
Snover - 120/124/100/124/120/80; harder-hitting mixed Weather setter. Too bad it basically dies to nearly every Ubers mon that's good, dying to PDon, Dialga, Xerneas with Geomancy, Mewtwo with both forms, and a long list of 350 cup-buffed mons like Pawniard, Mienfoo, Scraggy, Stufful, et cetera.
Vulpix-Alola - 76/82/80/100/130/130; Faster and can carry Aurora Veil. Still dies to a bunch of things, notably Steels including Solgaleo.
Abusers
Kyurem-White - 125/120/90/170/100/95; Hits like a truck with Blizzard, and you already know how hard it hits since it doesn't change in 350 cup! Can struggle with breaking a few of the superbulky walls like Mantyke, but they struggle to do anything back to you, so just keep clicking Blizzard until they send in one of the many base 100 or faster speed Steels like Pawniard.
Sandshrew-Alola - 100/150/180/20/70/80; Can double up for Aurora Veil, Swords Dance is pretty cool, Icicle Crash + Iron Head is cool, and can carry Rock Slide + Earthquake for great coverage thanks to being a Sandshrew still. Also can function as a Spinner, decent defensive typing thanks to Steel typing.
Cubchoo - 110/140/80/120/80/80; Only niche over Sandshrew is the possibility of running a Mixed set and a different possible movepool with Superpower, Play Rough, and Low Kick. Also doesn't have Icicle Crash, sadly :[
If I made any errors just remind me of how bad I am at making Smogon posts, its been a little while since I've played any serious 350 Cup games with anyone with a brain.