I'll wait until the full evolutions are revealed before making my decision, but going by the base forms, I like Litten the most.
Another thing that might influence my choice is the pokemon available in the new region, I mainly chose Chimchar in Pearl because of the only other fire types in Sinnoh were the Ponyta line (not that I don't like Chimchar, he grew on me as I played the game). I also rarely use water starters because of this, as there's usually a whole bunch of other water types to choose from.