I honestly love most the Pokemon introduced in Kalos, haha ;; However, there are some that stand out:
Sylveon: I love all Eeveelutions so Sylveon was already a fave when it was first announced. Love its design, type, shiny color palette, typing, HA, and movepool. It may not be considered the "best" Eeveelution, but it does pretty well in-game and is still adorable as heck (especially in Amie), hnnngh.
Aegislash: Cool design, cool pre-evos., cool typing (would've been better if not for the Steel nerf, tho), cool base stats, cool movepool. This is one of my new faves to use in competitive battling, especially since it can run a number of movesets. I'm also a fan of offensive Steel types because they can pack a punch while resisting a lot of types.
Carbink: This thing is so cute and I love its design and it being part Fairy and I wish it were a lot better than it really is ;; At least it can mutate into Diancie, which seems pretty good AND has a Mega Evo.!
Fennekin: Fave starter; it's absolutely adorable, is based off my all-time fave animal, and doesn't evolve into a Fire/Fighting type!
Greninja: Fave fully-evolved starter; love just about everything about it.