Favorite
Grass - Treecko - Obviously. But Chikorita comes in a close second. I actaully hate Chikorita 'cause in the anime, when Ash's Grovyle/Sceptile loves her (meganium), she rejects him, and act like a... I'm not even gonna say it.
Fire - Charmander - Yeah, i liked all of the fire starters (except for tepig), but charmander is just the best.
Water - Oshawott - Yeah, the only good gen V starter. Squirtle comes in with a close second.
Least Favorite
Grass - Snivy - rip-off of treecko. And a bad one at that. Seriously, it takes too long for this guy to set up. And he can't attack for his life.
Fire - Tepig evo line - tepig itself is cute, but when he evolves... Just no...
Water - NONE, i liked all of 'em
Overall
Favorite - Treecko
Least Favorite - Tepig