It's hard to pick just one, as many of my favorites are in this Gen, cuteness and otherwise... (Gardevoir, Altaria, Delcatty, Milotic, etc)
In the end, I'd probably go with Altaria because it can fly and it's freaking fluffy as hell.
Aguasjkga;wg or I'd go with Milotic, which, with recover, can be obnoxious to try and win against.
I basically killed it, however, with my Gardevoir that I've had basically the entire game.
I don't know why people prefer the starter pkmn, as I've never kept them on my team, or actually used them beyond getting them to evolve into their last form. (But I could just be a mega weirdo over here).