Here's to go more in-depth on my answer:
Sylveon has a base SpAtk of 110. With perfect IVs, maxed EVs and a beneficial nature, that's 350 Special Attack at Level 100. Now, to go more into the moveset:
Formula I used was (Base Power x Pixie Plate)STAB
Dazzling Gleam - Decent move, 80 Base Power, hits all adjacent foes. With Pixie Plate and STAB, this equals 144 Base Power. Very nice move to have. Learnt through TM99.
Moonblast - 95 Base Power with 100% Accuracy, and can lower target's SpAtk. Great move to have. With Pixie Plate and STAB this equals 171 Base Power. Learnt at Level 31.
Hyper Beam - HERE'S the big kicker. With Pixilate, the move is powered up 30% and changed to Fairy Type. This equals 195 Base Power. With Pixie Plate and STAB, this equals a whopping 351 Base Power with 90% Accuracy. This is Sylveon's main move. Learnt through TM15.
Draining Kiss - This gives Sylveon some recovery, healing by over half of the damage dealt. With Pixie Plate and STAB, this gives it 90 Base Power. Learnt at Level 20.