Chespin is my favorite of them. I picked it in my main copy of X, and it's the only time i've chosen the Grass starter in any of my main games. I've only picked it once other than that, and that was in my initial X Wedlocke (the Pokémon Center restrictions + Set mode run back in December that brought me to the verge of losing my mind and led me to terminate it in the middle of Victory Road). I like it myself design-wise the most, even if its special stats aren't top-notch.
Froakie i've used twice - in a Dark Monotype, where it did me pretty good for what it had. The other was my second X Wedlocke (an item restrictions one), where it survived to the end. Its Speed did me very well, and the Dark-type came in handy more than once. But I like Greninja's pre-evolutions more than it itself, but still pretty useful (had a Modest nature in that challenge, so it hit hard with special attacks).
Fennekin is the one i've used the least - in my first play of Y (a normal playthrough) and my Red Mono-Color, it had a bit to utilize but I wasn't the biggest fan of it. Probably my least favorite of them.
But in all honesty I really like all three of these starters, probably my favorite batch of all six generations.
