Okay, I'll throw in my two cents here.
Sylveon @ Choice Specs
Ability: Pixilate
EVs: 244 HP / 252 SpA / 12 Spe
Modest Nature
- Hyper Voice
- Psyshock
- Shadow Ball
- Hidden Power [Ground] / Hidden Power [Fire]
Sylveon is a really underrated Specs user in the current meta. The reason for this is some people that overlook her amazing base 110 Special Attack, great coverage, & solid Fairy typing. Her bulk isn't too bad either, so a Specs set is completely justifiable for her. Hyper Voice in conjunction with Pixilate equates to a base power of 117, & that's not even including STAB either, & it can put a dent in anything that doesn't resist it. Psyshock helps against Mega Venusaur, a very common switch in to Sylveon. Shadow Ball hits Aegislash for super-effective damage, while granting her nearly perfect type coverage. Hidden Power Ground rounds out her coverage by hitting & sometimes grabbing the OHKO on Heatran (Granted it doesn't carry a Balloon, or if it is popped beforehand. Hidden Power Fire is also a solid option & it has a good chance to OHKO standard Mega Scizor.
Here are some damage calcs to show you how good this thing is:
252+ SpA Choice Specs Sylveon Hidden Power Ground vs. 248 HP / 252+ SpD Heatran: 268-316 (69.6 - 82%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Choice Specs Sylveon Psyshock vs. 252 HP / 0+ Def Mega Venusaur: 194-230 (53.2 - 63.1%) -- guaranteed 2HKO
252+ SpA Choice Specs Pixilate Sylveon Hyper Voice vs. 252 HP / 0 SpD Skarmory: 187-221 (55.9 - 66.1%) -- guaranteed 2HKO after Leftovers recovery
252+ SpA Choice Specs Sylveon Hidden Power Fire vs. 252 HP / 244 SpD Mega Scizor: 308-364 (89.5 - 105.8%) -- 37.5% chance to OHKO
252+ SpA Choice Specs Sylveon Shadow Ball vs. 244 HP / 0 SpD Aegislash-Blade: 442-522 (137.2 - 162.1%) -- guaranteed OHKO
All these are common switch-ins to normal Sylveon sets, but if they come in on the wrong move, they are in for a world of hurt.