It's probably been brought up somewhere before, but specs Sylveon is worth another mention. A specs Hyper Voice hits harder than any other fairy move in OU, leaving a massive dent or outright OHKOing things expecting a hit from the defensive set.
Sylveon @ Choice Specs
Ability: Pixilate
EVs: 252 SAtk / 248 HP / 8 Spd
Modest Nature
- Hyper Voice
- Shadow Ball
- Hidden Power Fire/Ground
- Psyshock
This set has a lot of surprise factor to it, being able to lure in your common checks and demolish them with a super-effective hit. Hyper Voice is what you'll want to use 90% of the time, hitting single resists harder than anything not super-effective. Everything else is there only to hit fairy resists super-effectively. Shadow Ball 2HKOs Aegislash through shield form, while a +2 shadow sneak from weakness policy sets won't be able to kill you. The choice of hidden power is determined by what you're looking to bait in. HP ground 2HKOs specially-defensive Heatran, while HP fire hits Scizor and Ferrothorn. HP rock is also an option for things like Volcarona and Rotom-H, but neither of them are seen nearly as often. Psyshock hits Mega Venusaur decently hard, though it falls short of 2HKOing physically defensive sets while Sludge Bomb is a 3HKO. This is probably the most flexible spot on the moveset, as Psyshock doesn't hit anything else besides Tentacruel and Scolipede. Draining Kiss can be used to get some health back from weakened targets, but it will likely force you to switch out once something healthier comes in. If Sylveon gets trick or something from a tutor in the near future, it would be an excellent way to ruin the pink blobs as they try to come in on you.
The set does require a bit of prediction to pull off sometimes, but it's definitely worth using.