Where the fuck did all of the trolls and shitposters come from all of a sudden? Fuck off people, you're annoying.
OP, Sylveon's most popular set is a Cleric/wishpasser.
Leftovers
252 HP / 4 SpA / 252 Def or SpDef
Bold or Calm
Pixilate
Hyper Voice
Wish
Protect
Heal Bell
Pixilate Hyper Voice is a must, no matter which set you're running. It might be a little harder to get a hold of ingame, but its well worth it.
You could try Calm Mind or Specs, too.
The CM set is the same as above, but runs Hyper Voice/Moonblast, CM, and one of Sub, Psyshock, or Shadow Ball.
I'm not sure what the best build is for Specs sets, actually. I know that you want this:
Sylveon @ Choice Specs
Modest
Pixilate
252 SpA
Hyper Voice
Psyshock
Shadow Ball
But I'm not sure about the other EVs or the fourth moveslot. I think Baton Pass is a decent option for pseudo-U-turns, and 252 Speed puts you ahead of the Azumarill Speed tier.
Hope this helps. ^_^
