Favorite: Samurott, because its appearance, cry, and fighting abilities give me the impression that it can beat any non-legendary opponent. Sometimes I do wish its Special Defense and Speed were higher, however. Then again, it's all just to balance things out.
Least favorite: Meganium, because its movepool doesn't appeal to me. That's not saying that I dislike it overall, but I felt like I had to choose a least favorite.
Favorite: Torchic. Reason: The entire Torchic line is awesome in both design and battling ability. The original Fire/Fighting-type line as well. Torchic itself is so adorable, while Blaziken is just totally awesome and sweet. Treeko and Turtwig probably my next favorites, due to their useful moves and cool designs.
Least Favorite: Chikorita. Reason: Totally useless, no real strengths, weak moves, designed poorly, it's the pits.
Oh, Cynthia. Where for art thou Cynthia? Heheh. Seriously, where are you!?
Best Game Characters of all Time: Ryu 3, Nina 3, Ryu 2, Nina 2, Rei, Ratchet, Talwyn, Princess Pride, Megaman.EXE, Elecman.EXE, Bass. EXE, Tails, Geo, Sonia, Pit
Best Anime Characters of all Time: Van, Fiona, Irvine, Cynthia, Yugi, Tea, Joey, Dark Magician Girl, Kuriboh, Yuma, Astral, Number 39: Utopia, May, Inuyasha, Miroku, Sango
I apply Felix's Law: "Smile, tomorrow could be worse."
Favorite: Oshawott. I absolutely love all of its evolutions, especially Dewott. It is by far the best starter in Gen 5, which was severely lacking in good water types.
Honorable Mentions: Squirtle, Piplup, and Torchic
Least Favorite: Snivy. I used one, and it was completely useless. I'm not a fan of the design of it at all, and I hate its movepool (although that did get better with the move tutors)
Honorable Mentions: Totodile, Bulbasaur, and Turtwig
For aesthetic reasons: Bulbasaur (favorite), Turtwig (least favorite)
For in-game reasons: Torchic (favorite), Tepig (least favorite)
For competitive reason: Chimchar (favorite), Squirtle (least favorite)
EDIT: And Piplup gets an honorable mention because he is just so dang cute. And Empoleon actually kicked some butt when I used him for my Grass/Steel team.
Jumping on the Fenne-Bandwagon
My most favorite is Piplup. First of all, I love it's typing; water/steel; great coverage and resistances. Second, I adore Dawn's Piplup in the anime. He is super cute and adorable with a swag!~
And for so many reasons, Piplup is one of the greatest pokemons ever (that includes it's evolutions).
My least favorite is Tepig. Like someone else said here, it had a lot of potential but Emboar totally ruined it for me. Tepig has a great design until it got weirder and messed up. Emboar looked like a roasted Snorlax.
Kenta's Pokemon Party