Grass: Serperior, as an owner of snakes of course this is my favorite and my favorite grass type in general. Too bad it's not a combo of poison, but grass snakes aren't, *sigh*, venomous. Least favorite? Turtwig...prolly. Torterra grew on me but all the Sinnoh starters were dumped quite quickly in the PC after getting a Shinx.
Fire: Charizard, my goodness duh! It's a flying fire breathing lizard. Least favorite? Chimchar...I hate monkeys.
Water: Tie between Feraligator an amazing alligator and Samurott a samurai...um, badass thing! Least favorite? Piplup hands down. Thank you Dawn's Piplup for ruining that Pokemon for me. Your constant stupidity and unfunny antics in the cartoon completely makes me hate it now. Least favorite is Froakie hands down now.