Long Post. All potentially viable pokemon; based in Pokedex order; GEN I.
(sets with a strikethrough) are valid thoughts for sets, but outclassed completely be a different mon.
Bolded sets/mons are suspected A-tier (central to meta) stuff.
Italicized sets/mons are questionably viable; suspected D-tier.
Venusaur (Overgrow/(Prankster/Flower Veil/Aroma Veil)) - Defensive Subseeder, comparable to Tangrowth.
Venusaur (Chlorophyll/Desolate Land) - Chlorophyll Sweeper, Earthquake
Venusaur (MEGA)
Charizard (Solar Power/Desolate Land) - Harder hitting, but frailer than Mega-Y. Takes similar residual damage to a burn. Consider for wallbreaker or fast/frail sweeper stats, speed tier is poor though.
Charizard (MEGA - X)
Blastoise (Rain Dish/Regenerator) - Defensive Spinner, probably outclassed by Tentacruel in this role, but not non-viable. Offensive stats too low to run a true offensive set, but consider yawn or scald.
Butterfree ( (Tinted Lens/Compound Eyes) / Prankster) - Completely outclassed by Vivillon.
Butterfree ( Tinted Lens / Adaptability ) - Probably too frail to run this set. Consider Venomoth instead.
Raichu (Lightning Rod / Sheer Force) - Sheer Force boosts Thunderbolt and Focus Blast; giving it more offensive presence without needing Nasty Plot setup.
Raichu (Lightning Rod / Compound Eyes) - Makes Thunder and Focus Blast fairly accurate.
Raichu (Lightning Rod / Prankster) - Bit gimmicky, prankster allows ease of setting up sub and getting in a Nasty Plot. set needs to come in vs Stall or Wall to work though as Prankster sub wouldn't help against offensive.
Sandslash (Sand Rush / Regenerator) - Defensive Spinner. Mostly outclassed by Excadrill.
Nidoqueen (Poison Point/(Prankster/Aroma Veil) - Although Nidoqueen can run all of Nidoking's sets, pretty much Nidoqueen's better at a defensive Toxic stall set than King. (Rocks, TSpikes, Sub, Roar).
Nidoking (Sheer Force / Protean) - Self Explanatory, wide variety of wallbreaking coverage moves now with stab.
Nidoking (Sheer Force / Speed Boost) - Improves speed tier to allow it to try to sweep; without Protean/Life Orb it might not hit hard enough to pull this off though.
Clefable ((Magic Guard/Unaware)/(Sheer Force/Protean/Life Orb)) - Offensive CM set. (If Magic Guard you can just run a normal life orb).
Clefable (Magic Guard/(Unaware/Multiscale/(Filter/Solid Rock))) - True Wall Set.
Clefable (Magic Guard/Regenerator) - Wish Passer.
Clefable (Magic Guard/Aroma Veil) - Immunity to Taunt makes it better as a status spreader and at Wish-Tect stall.
Clefable (Magic Guard/(Pixilate/Aerilate/Refrigerate)) - Boost Hyper Voice into something more powerful/accurate than normally available moves. Aerilate in particular would be surprising coverage.
Ninetales (Drought/Chlorophyll) - Fastest thing with Sun + Chlorophyll available. Would want to use NP once and attempt to sweep as long as possible. Hard Countered by Primordial Sea, obviously.
Ninetales ((Drought/Flash Fire)/Magnet Pull) - Trap and setup on steels.
Ninetales (Flash Fire/Prankster) - Will-o-Wisp, Sub Pranks. Probably outclassed by something with WoW and recovery.
Wigglytuff (Competitive/Weak Armour) - Valid thought, but the speed tier is probably too low to be viable.
Wigglytuff (Competitive/Regenerator) - Wish Passer. Consider Blissey or Clefable instead.
Vileplume (Chlorophyll/Desolate Land) - Chlorophyll sweeper, Dazzling Gleam, but no Growth.
Vileplume (Effect Spore/Flame Body) - Double Contact ability physically defensive.
Vileplume (Effect Spore/(Prankster/Flower Veil/Aroma Veil)) - Aromatherapy, Leech Seed Cleric. (Probably outclassed by Blissey, but has unique typing).
Parasect (Dry Skin/Primordial Sea) - Similar recovery to Poison Heal, but fairly bad bulk and double flying weakness. Sub-Seeder.
Parasect (Effect Spore/Prankster) - Prankster Subseeder. Probably outclassed by Breloom. (Can't use other defensive abilities because too frail/slow).
Venomoth (Tinted Lens/Adaptability) - Tinted Lens makes Bug/Poison a perfect stab pair (against all but non-mega Aggron). Adaptability vastly boosts the STAB. Quiver Dance cleaner.
Dugtrio (Arena Trap/Magic Bounce) - Trap a stallmon and wear it down or setup.
Golduck (Swift Swim/Primordial Sea) - Not terrible, but fairly mediocre.
Golduck (Cloud Nine/(Sheer Force/Protean)) - Pseudo-Nidoking (similar library of coverage moves; similar base SpA and Spe tier).
Primeape (Defiant/Weak Armour) - Weak Armour Triggering activates defiant too giving you great Atk and Decent speed. Unfortunately due to bad bulk and speed tier, you'd need to invest in Speed AND bulk to pull this off meaning no attack investment and questionable viability.
Primeape (Defiant/(Speed Boost/Protean)) - Outclassed by Darmanitan and Infernape.
Arcanine (Intimidate/Regenerator) - Intimidate, WoW/Attack leave.
Arcanine (Intimidate/Desolate Land) - Helps offensively and defensively (Better Morning Sun Heals).
Arcanine (Intimidate/Magic Guard) - No Recoil or Rocks damage; Better defensively as well.
Arcanine (Intimidate/(Prankster/Aroma Veil)) - WoW, Sub, Morning Sun; stall set.
Arcanine (Intimidate/Protean) - Better use out of coverage.
Arcanine (Intimidate/Magnet Pull) - Allows revenge-killing steels more easily.
Arcanine (Intimidate/Mold Breaker) - Ignores anything's Flash Fire...mostly for killing Ferrothorn.
Arcanine (Intimidate/Refrigerate) - gets better coverage on ESpeed to revenge kill etc.
Arcanine (Intimidate/Speed Boost) - Attempts to be more of a sweeper than a defensive mon; probably non-viable (too low base attack to sweep).
Poliwrath (Swift Swim/Primordial Sea) - Fast semi bulky but weak or frail, but strong (Belly Drum); Swift Swim user.
Poliwrath (Water Absorb/Prankster) - Rest, Sleep Talk, Circle Throw. Bulky Prankster Phasing. Mostly outclassed by similar Throh set, but has unique typing.
Alakazam (Magic Guard/(Sheer Force/Protean)) - Frail Offensive sets.
Machamp (No Guard/Speed Boost) - Uses Dynamic Punch's confuse to setup (Bulk Up/PuP) and tries to sweep with Speed Boost speed.
Machamp (No Guard/(Hustle/Scrappy)). - Wall-breaking set.
Victreebel (Chlorophyll/Desolate Land) - Compare to Venusaur. Frailer; but harder hitting and gets weatherball.
Tentacruel ((Rain Dish/Liquid Ooze)/Regenerator) - Defensive Spinner.
Tentacruel (Rain Dish/Primodial Sea) - Similar recovery to leftovers and stronger Scald/Hydro Pump; probably inferior to the Defensive set.
Tentacruel (Liquid Ooze/Serene Grace) - Great Scald Burn Chance; can also run Waterfall for flinch, but fairly meh.
Golem ((Rock Head/Sturdy)/ Protean) - Golem's stats are lower than similarly-typed Rhyperior. Only niche it has over it are Sucker Punch and Explosion. Protean gives you STAB in these and gives you better defensive typing until their usage.
Golem ((Rock Head/Sturdy)/ (Refrigerate/Pixilate/Aeilate) - Give Explosion a better typing.
Slowbro (Regenerator/(Natural Cure/Multiscale/Unaware) - Standard defensive Slowbro.
Slowbro (Regenerator/Intimidate) - Increases value of switching in and back out.
Slowbro (Regenerator/Serene Grace) - TWave/Trick Room + Flinch abuse or higher burn chance from Scald.
Slowbro (MEGA)
Dodrio (Tangled Feet/Serene Grace) - This is horrible but I'm adding it so I'm not startled when I see someone using it. Tries to paralyze with Body Slam, then confuse self with Thrash to double own evasion. Yea. No.
Muk (Poison Touch/Speed Boost) - Mostly Inferior to Toxicroak and Garbodor, but STAB Gunk Shot and Poisonchance on all coverage moves.
Cloyster (Skill Link/Technician) - Classic offensive set.
Cloyster (Skill Link/ (Pixilate/Aerilate)) - Gives Spike Cannon a different coverage type.
Cloyster ((Overcoat/Shell Armour)/Regenerator) - Defensive Spinner/Spikes.
Cloyster (Skill Link/(Speed Boost/Mold Breaker)) - Offensive Spinner/Spikes.
Gengar (Levitate/Sheer Force) - Similar offensive potential to normal life orb set without the life orb recoil.
Gengar (Levitate/Prankster) - Prankster Taunt and Destiny Bond.
Hypno ((Insomnia/Inner Focus)/Speed Boost) - Gathers speed and Baton Passes it. Specially Defensive; runs some physical offense (Drain Punch, Zen Headbutt).
Hypno ((Insomnia/Inner Focus)/Prankster) - Prankster helps it Baton Pass Belly Drum or Nasty Plot.
Kingler (Sheer Force/Speed Boost) - Poor Speed Tier, but great variety of physical coverage. Would run SD and get the Speed Boost, boost to pseudo-Dance to get to very high attack and decent speed tier.
Exeggutor (Chlorophyll/Desolate Land) - Probably inferior to Victreebel and Venusaur normally; but the unique typing will give it a niche on some teams.
Hitmonlee (Reckless/Tough Claws) - Offensive Spinner/Wallbreaker.
Hitmonlee (Reckless/Magic Guard) - Mixed Spinner. (Immune to hazard damage and crash damage from HJK; also less powerful than the Tough Claws set).
Hitmonlee ((Reckless/Limber)/Speed Boost) - Attempts to stay in and sweep. Probably inferior to Mega-Lopunny and Mega-Medicham.
Hitmonchan (Iron Fist/Tough Claws) - Bulky Offensive Spinner. Carries Rapid Spin and Drain Punch.
Weezing (Levitate/Prankster) - Too frail to be normally defensive; but Prankster WoW, Pain Split, Taunt, TSpikes can manage to do something. Mostly outclassed by Cofagrigus.
Chansey (Natural Cure/Eviolite) - Normal Chansey. Boosted by KO being rarer in this meta.
Kangaskhan (Scrappy/Speed Boost) - Uses Fake-Out to get a Free Speed Boost and uses Fighting + Normal STAB. Bulkier, but weaker than MLop and doesn't use a mega-slot. Think of as pseudo-MLop.
Kangaskhan (Scrappy/Tough Claws) - Hits harder, but is slower than previous set; consider sucker punch for priority.
Starmie (Natural Cure/Regenerator) - Mixed Spinner (Offensive/Defensive).
Starmie (Analytic/Primordial Sea) - Boosts Hydro Pump/Scald, boosts Thunder's accuracy and may force switches out of mons that like other weathers.
Starmie (Analytic/(Sheer Force/Protean/Adaptability)) - Straight forward offensive spinner
Scyther (Technician/Aerilate) - Technician AND Aerilate boosted Quick Attack should slay after a SD.
Scyther (Technician/Magic Guard) - Standard Scyther with immunity to rocks.
Scyther (Technician/(Refrigerate/Pixilate)) - Coverage choice!
Jynx (Dry Skin/Compound Eyes) - Runs (Lovely Kiss, NP, Blizzard, Focus Blast/Psychic); semi-viable because of the ICE Stab, but fairly poor speed tier.
Jynx (Dry Skin/Snow Warning) - Perfect accuracy Blizzard. Inferior to the compound eyes set in all but the residual damage, however.
Pinsir (Moxie/(Speed Boost/Tough Claws/Protean)) - Mostly inferior to Heracross, but isn't quite as weak to flying. Basic offensive set.
Pinsir (MEGA)
Tauros (Sheer Force/Protean) - Self-explanatory offensive set.
Tauros (Sheer Force/Mold Breaker) - Ignores Levitates and Flash Fire's (Fire Blast to hit Ferro is a popular coverage option).
Gyarados (Moxie/Aerilate) - Gives access to flying stab and boosts attack on KOs.
Gyarados ((Intimidate/Moxie)/(Volt Absorb/Motor Drive)) - Covers the electric weakness (and heals/boosts speed when foe tries to hit you with it). Can be defensive or offensive (use to setup DD).
Gyarados (Intimidate/Regenerator) - Defensive switch in/out. Probably outclassed by Intimidate Slowbro.
Lapras (Hydration/Primordial Sea) - Rest only taking one turn gives you amazing bulk opening up a lot of viable sets, DD + Waterfall, Perish Trap, Normal Specially Offensive (though save Freeze Dry Special Lapras < Manaphy).
Lapras (Water Absorb/No Guard) - Perfect accuracy Sleep (Sing) and Blizzard.
Ditto (Imposter/ (Speed Boost/Unaware/Filter/Tinted Lens)) - Ditto wants to use an ability that makes it better than the thing it copies (Faster, ignore foe's stat boosts while they deal with yours; able to hit a foe with stab that can only use coverage on itself, etc.).
NOTE: Imposter/Infiltrator will NOT allow you to copy the foe through a sub.
Vaporeon (Hydration/Primordial Sea) - Besides it's use as a cleric (Wish/Heal Bell); it's inferior to Manaphy; but the Wish/Heal Bell is enough to be a niche.
Jolteon ((Quick Feet/Volt Absorb)/(Refrigerate/Pixilate/Aerilate)) - Choice Coverage Hyper Voice. Refrigerate is probably the best new coverage choice; the other two can startle some players though.
Jolteon ((Quick Feet/Volt Absorb)/Sheer Force) - Boosts power of Thunderbolt and Shadow Ball.
Flareon (Guts/Prankster) - Unique typing for guts mons; Rest/Talk with prankster makes up for speed tier.
Omastar (Swift Swim/Primordial Sea) - Super Fast Special Sweeper.
Omastar (Weak Armour/Competitive) - Foe sets you up by hitting you. (+2 SpA, -1 Def, +1 Spe)
Omastar (Shell Armour/ (Storm Drain/Levitate/Sap Sipper/(Sturdy/Sash))) - Defensive ability to help Shell Smash Sweep.
Kabutops (Swift Swim/Primordial Sea) - Fast Physical Sweeper. (Swords Dance Setup).
Kabutops (Weak Armour/Defiant) - Foe sets you up by hitting you. (+2 Atk, -1 Def, +1 Spe)
Kabutops (Battle Armour/Sap Sipper) - Defensive ability to that boosts attack. Allows to bait grasses (X-Scissor).
Aerodactyl (Rock Head/Aerilate) - Fast Frail Revenge killer/Cleaner. (Competes with Talonflame (similar usage)).
Aerodactyl (MEGA)
Snorlax (Thick Fat/Serene Grace) - Paraflinch Abuse.
Snorlax (Thick Fat/(Magic Bounce/Flame Body)) - True Defensive set.
Snorlax (Immunity/Water Veil) - NO Burns/Poison!
Snorlax (Thick Fat/Scrappy) - Make Fighting/Normal perfect coverage.
Snorlax (Thick Fat/Early Bird) - Shorter Rests eliminates the need for sleep talk and better CurseLax.
Snorlax (Thick Fat/Prankster) - Prankster Sleep Talk to improve the speed tier.
Articuno (Snow Cloak/(Magic Guard/Filter)) - Fairly low viability. With Magic Guard works as a Defogger, otherwise decent special wall/attacker with bad defensive (but good offensive) typing.
Zapdos (Static/Flame Body) - Two contact abilities make for a good chance of disabling a foe that touches you.
Zapdos (Pressure/No Guard) - Uses Zap Cannon.
Zapdos (Static/(Regenerator/Magic Guard)) - Makes a more solid defogger/defensive mon.
Zapdos (Static/Delta Stream) - Removes weaknesses.
Moltres (Flame Body/Static) - Two contact abilities make for a good chance of disabling a foe that touches you.
Moltres (Flame Body/Magic Guard) - Deals with Stealth Rock.
Moltres (Flame Body/No Guard) - Hurricane doesn't miss.
Moltres (Flame Body/Gale Wings) - Priority Hurricane.
Dragonite (Multiscale/Delta Stream) - Defensive.
Dragonite (Multiscale/Aerilate) - STAB ESpeed.
Dragonite (Multiscale/Tough Claws) - Boosts most Dragonite moves.
Dragonite (Multiscale/Protean) - STAB in a variety of coverage moves.
Dragonite (Multiscale/Primordial Sea) - Hurricane and Thunder.
Dragonite (Multiscale/Magic Guard) - Guarantees max HP on switch in prevents toxic damage.
Mew (Synchronize/Natural Cure) - Ultimate Status Absorption.
Mew (Synchronize/Prankster) - Variety of utility, WoW; Transform; Defog; Sub/Roost
Mew (Synchronize/Magic Bounce) - Ultimate switch in/defensive pivot.