God Tier:
-Mudkip (No, not because of that stupid joke. Swampert's a freakin' beast)
-Chimchar
-Bulbasaur
Swampert is without a doubt the best starter, seeing as it can hit like a ton of bricks and take numerous amounts of pain and still keep going. Infernape is fast and wrecks an unprepared team. Venusaur does the same, but more in the UU environment.
Good Tier:
-Piplup
-Totodile
Empoleon's good enough to make it to OU standings but can be inconsistent. Feraligatr wrecks in UU but isn't the Scizor of UU if you know what I mean.
Mid Tier:
-Torchic
-Treecko
-Turtwig
Blaziken's a solid fighter but is completely outclassed by Infernape, and is simply too slow to do much. Sceptile is simply too predictable, and while it's strong, there are better Pokemon to do the job it can do. Torterra is just sort of bland. Grass/Ground isn't a great type combination...
Low Tier:
-Cyndaquil
-Charmander
Both of the Fire starters without a secondary type that resist Rock simply die to Stealth Rock. That's all there is to it, really...They're fine otherwise but lacking neutrality to Rock really, really hurts them. I would also put Charizard in the lowest tier if not for the fact that it can actually do something, which is more than what can be said of Typhlosion, who simply looks cool.
Lowest Tier:
-Chikorita
-Squirtle
Meganium is just useless on all fronts. There are better Aromatherapy users like Roserade out there, and Blastoise is just useless period in the sea of bulky Water-types that can't do much else but sit there and be pretty (Vaporeon laughs at all of you!).
Anyone who's ever attempted making actual battle teams should or will agree with me on this one...