Sylveon by far, but I'm really liking the new guys overall.
I ran with Aegislash, Talonflame, and Trevenant (though Phantump is way cuter) and I enjoy all of them a ton. Just so fun to play with. Though, I've never had such a hard time picking a story team before. I must've switched teams completely like 10 times before it was all over. There's just so much that's offered.
It was hard to pick just one for a favorite, but Sylveon has got me through so much, is so powerful, and is just plain adorable too.