I agree with Vaporeon -> B, Mega Pidgeot -> A-, Quag/momo -> B-, Sylveon -> S, Aero -> A+, Thundy-I -> B+, M-Sab -> B+, and Bibarel -> C
I'd like to nom Mega Venusaur for A-/B+. ellipse has convinced me that this thing is pretty damn good, so you can blame him if you disagree. Thundy was the biggest thing holding it back by far, and with that gone Venusaur checks a really large portion of the metagame. It's the one of the hardest counters available for Sylveon, Keldeo, Serperior, Ferrothorn, Rotom-W, Manaphy, and Mega Blastoise. It's a very good check to M-Lopunny, Ursaring, Mega Diancie, Terrakion, (M)-Slowbro, Slowking, Tank Chomp, Mega Gyarados, and Tyranitar. I think the best set is some variation of
the coil set this:
Venusaur @ Venusaurite
Ability: Chlorophyll
EVs: 248 HP / 96 Def / 144 SpD / 20 Spe
Bold Nature
IVs: 0 Atk / 30 SpA / 30 Spe
- Spore
- Synthesis
- Seed Flare
- Hidden Power [Fire] / Sludge Wave
This is the standard defensive spread from OU, so it could probably be tweaked. It does avoid the 2hko from Keldeo's Specs Steam Eruption after SR and 1 turn of Burn damage (so you can at least trade with it or stall Steam Eruption for Synthesis). It might be worth scrapping the Speed EVs for more Def, since things tend to hit harder and faster in STABmons than OU. HP Fire is better in my mind, since the pokemon that switch in on HP Fire are taken care of by Spore anyways (Dragons, Fire types, and Flying types in general). Seed Flare hits pretty hard even with no investment, thanks to its 120 BP and 122 SpA, and SpD drops can break a lot of slower checks, like Chansey. It's issue is mostly that it is susceptible to getting whittled since it switches into a lot of scalds and steam eruptions, doesn't have passive recovery, and can get PP stalled out of Synth (although that's less of an issue in STAB since so many things run 8 PP moves and the games go by pretty quick).
I'd also like to nom Jolteon for B+. It's tied the fastest unboosted non-mega pokemon, and generally outspeeds everything. It has excellent coverage in TBolt + Ice Judge, and has lots of options between Sub + Electrify, Volt Switch, HP Fire, or whatever random normal utility move you want to stick on it (mostly emergency Rapid Spin or Heal Bell). The two best sets that I've seen are Sub + Electrify and Fissure's 4 Atk set. The 4 Atk set is a solid pivot, which can weaken teams in the early- or mid-game. It suffers a bit late game from losing the speed tie against other base 130s due to HP Fire. The electrify set tends to just win outright in the late-game, since it can easily PP stall checks with Sub + Electrify (which has 48 total PP, while most pokemon are running lots of 8 PP moves in STABmons), this is especially easy if you get a Sub up for free (all you have to do is click Sub a bunch, then click Electrify a bunch, and get 1 mixup in).
Jolteon @ Icicle Plate
Ability: Volt Absorb
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk / 30 SpA / 30 Spe
- Volt Switch
- Thunderbolt
- Judgment
- Hidden Power [Fire]
Jolteon @ Icicle Plate
Ability: Volt Absorb
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Substitute
- Electrify
- Judgment
- Thunderbolt