The plant one, according to the interwebs.
Thing is, I like fennekin the most as well as it's evolved form, but it's final form the least of the three. I'm also grabbing charmander in Lumiose so having two fires would be redundant.
Chespin is in the middle of the pack, while I find its evolved form the ugliest and its final form my favorite but it's pretty slow.
Froakie is my least favorite starter while its middle form is in the middle of the three along with its final form but I find its whole line uninteresting despite the greater arsenal it has compared to the other two.
In any game, I recommend playing for end game.
Fennekin's line has the least arsenal variety compared to the other two as well. Very hard choices. :(
If I do any sort of endgame planning, it probably won't include any starter I pick.