For X-Y, I'm leaning toward Froakie. Chespin's adorable and Fennekin looks cool, so I'm kinda digging "dorky" looking Froakie. I'm thinking he'll be like Oshawott in that he may be dorky now, but he'll grow up totally awesome.
On the other hand, I agree with Vencoris that, at least in my experience, it's hard to find cool, or at least decent, grass types in the wild, whereas good water and fire types are far easier to encounter in the wild, like Gyrados or Magmar for example. So Chespin's an option for me too. Unfortunately, grass type starters have been hit or miss. Popular opinion seems to be that Bulbasaur was decent, Chikorita sucked, Treecko was an improvement (esp. in evolutions), Turtwig was supposedly the best grass starter (esp. in final evolution), and Snivy looked cooler than he performed. Like I said, I haven't played a Pokemon game since Crystal, so I'm just going by what I've read everywhere on the 'net. (I chose Cyndaquil as my starter in Crystal.)
So, yeah, I'm probably going either Froakie or Chespin. And I'm probably going to go with Y over X because as cool as that legendary deer looks, I'm more interested in a legendary flying type.
EDIT: Hathen, it's funny you say that, because from what I understand Charmander was at a disadvantage on the first two gyms yet Bulbasaur did well. Yet in the next set of games, Chikorita was the liability for the first couple of gyms.