I'll try to put them in my own tiers...
Easy: Torchic (obviously), Froakie (again, obvious), Cyndaquil (for that Eruption), Mudkip, Chimchar, Turtwig (since y'all like him so much), Snivy (Contrary FTW), Tododile (Sheer Force is beautiful)
Normal: Piplup (Defiant is pretty nice for a Mixed set), Charmander (although he seems kinda Borderline here), Bulbasaur (some may disagree), Squirtle, Fennekin, Chespin (even though I love him), Treeko (Stats are good, but movepool is lacking)
Hard: Tepig, Oshawott, Chikorita (this one here is probably on the verge of falling off Hard into Insane)
The ones you can get ingame now are (they'll all be Events sooner or later, but the most powerful will obviously be later in the game):
Chikorita, Tepig, Mudkip, Piplup, Chespin, and Cyndaquil. Hopefully I'm not missing anyone.