I typically get attached to the designs of Pokémon that are based on inanimate objects, so naturally some of my favorite Kalos Pokémon include Honedge (and Doublade and Aegislash), Bergmite (and Avalugg), and Klefki.
Klefki is actually one of my favorite Fairy-types in general if only because I was generally indifferent to the Fairy type upon its introduction, and one of my friends, whose favorite type is now the Fairy type, utterly hates Klefki, so I decided to pretend to like Klefki to play around with her, but when it comes down to it, I found that I do actually like Klefki's design and theoretical Prankster shenanigans.
Beyond that, Chespin is my favorite starter, and I tend to choose the Grass-type starter anyway, so that works. Chesnaught has a somewhat un-limber design, but I like the theme that Chesnaught got more than what Delphox and Greninja got, and it's always fun to have a Grass-type that can learn a Rock-type move.
My favorite legendaries would pretty much be Xerneas first, Zygarde second, then Hoopa third. If I was considering all of Zygarde's forms, then Zygarde would probably not even make the list.
I chose X because I liked Xerneas more than Yveltal. Everyone liked to go towards Y because it seemed cooler, but I got attached to Xerneas' design, and now Xerneas is one of my favorite Fairy-types.
I like Zygarde because the Ground- and Dragon-typing is cool, and the fact that Game Freak made a one-eyed snake for a legendary Pokémon is kind of funny to me.
Hoopa's just a cute-looking, mischievous little guy. The Ghost- and Psychic-typing is a bit worrying for Dark-type coverage, but neat either way.