I voted Blastoise for nostalgia(it was my first starter ever in my very first Pokemon game, Red); but I have all kinds of favorites.
Kanto- Charizard is a very close second to Blastoise, espeically its shiny form.
Hoenn- Blaziken. I can't explain it, but it just looks so cool to me.
Sinnoh- Infernape. It's like the pokemon version of Dragonball Z. I even nicknamed mine "Gohan" in Pearl(It's the only one on my team I remember after giving my Pearl to my little cousin because I was disappointed with that game. Platinum was a huge improvement).
Unova- Serperior. It's a grass snake. Too awesome.