Favourite: Turtwig. So cute, and it's evolved form has an awesome design both visually and conceptually, plus a neat dual typing.
Least Favourite: Individually, it's Oshawott without a doubt. It looks like something a whale barfed up, and is really annoying in the anime (though it's evolved forms are my favourite Water starters). As for whole lines, I'd have to say the Charmander family. Overrated as hell, and horrible in battle.