Now that we have all the evidence ("Wait, I object!" "Overruled!" "I object!" "QUIET!" "Dang!") re: these starters, what they evolve into etc, I can actually give some thought to this question.
Frankly, I found all the starters fairly lacklustre when they were first revealed and so my opinion on them was to be heavily influenced by what they'd end up being. The second forms didn't help much (Frogadier was pretty awesome looking but the other two were rather odd) - and the final forms... well.
Strictly design-ways, Delphox is my favorite on account that the other two have something that completely screws them over. Chesnaught's anatomy is extremely awkward - I get what they were TRYING to do with it but it kind of didn't work, and Greninja's god damn tongue scarf completely ruins it for me. Not that Delphox is perfect either (those ear tufts, man @_@), but I like the wizardy direction they went with it, it looks good in art (even if a bit less so in the actual game) and the type combination (I've wanted a non-legendary non-useless Fire/Psychic for quite a while now!). Not bad for something that ends up evolving from a Bobble-headed Bootleg Zorua clearly created to appeal to kawaii-shrieking 13-year-old banshees.
I've yet to properly use any of them due to lack of X/Y but Protean Greninjas are all over the place on battle simulators and are kind of annoying as hell to deal with, whereas I've yet to see a single Delphox or Chesnaught - possibly because there tend to be better options for their type combination and they DON'T get the benefit of being Protean pains-in-the-ass.
... To be fair, though, I suspect that if and when I start a run of X/Y I might not really bother with the Kalos starter altogether. I might grab Chespin simply for the sake of having my roaming legendary bird being Articuno and because I nearly always do my first run through a region with the Grass starter and not really do much with it as I have every intention of running a Mega Blaziken on my Kalos team. But hey, might do a fire-type prisoner exchange and hurf a Fennekin into Alpha Sapphire/Omega Ruby if such would be possible? XD