Gourgeist-Small @ Leftovers
Bold (+Defence, -Attack), Insomnia
252 Defence, 252 HP, 4 Sp. Def
-Will O' Wisp
SubSeeder Gorgeist is just evil. Its tying is simply godly, lending it a plethora of useful resistances and immunities. Unfortunately, especially in his Small form, he lacks outright hitting power and has to resort to chipping opponent's health away with burns and Leech Seed. That said, he's amazing at it. With Sub+Protect and Leech Seed active, he becomes practically unkillable. Not to mention Delphox aways plays carefully around him in fear of switching into a Shadow Claw (even if you're not running it... Lower ladder for the win) and stuff like Heatran hasn't been released yet. Trick Or Treat is a neat gimmick, but generally just not worth the moveslot it occupies. The only niche I can think of is removing Leech Seed immunity from Grass types, but even then they'll just switch out anyways. Could have a niche use on hazard stacking teams, but with the Defog buff I doubt hazard stacking teams will be that popular anymore.
Gengar @ Gengarite
Timid (+Speed, -Attack) - Levitate
252 Speed, 252 Sp. Atk, 4 HP
-Shadow Ball / Disable / Pain Split.
So let me tell you a little story. One time, some jerk off at Gamefreak had just gotten back from a party when an idea struck him. Maybe it was the copious amounts of drugs he had been ingesting prior, or maybe he's just plain stupid. The world may never know. Anyways, his idea was to take one of OU's best Pokemon, hugely increase its Speed and Special Attack, give it a reason to use its Poison STAB, and give it Shadow Tag. The result is this monster. Really, Mega Gengar is pretty simple. Get him in, set up a Substitute if necessary (Substitute stops Aegislash and Mega Mawile from revengekilling him) and blow stuff away. Sludge Bomb actually becomes the STAB of choice because of its higher base Power and its Poison chance (which lets you beat walls like Blissey), and Focus Blast hits Steels and stuff. Shadow Ball can be carried for even more STAB fun, or Disable to beat Mega Khangaskan, or Pain Split just cause.
I'm not even going to list a set for this one, there's like 5 of them that all work effectively.
Everyone's favourite robot death bug is back and better than ever. While Genesect got nothing new and even lost a resistance to Ghost and Dark, he serves as a very good check to a lot of the top threats in the formative metagame. In fact, the nerfs might actually work in his favour because they might give him just the hold he needs to stay in OU this time around. For sets, he's got so many options that it's hardly practical to list them all. Among my favourites are the classic Scarf scout, physical Scarf scout, Rock Polish, and status lure. Physical Scarf scout in particular works great firing off absurdly powerful U-Turns and nailing Fairies with Iron Head, and can even blow up a threat with a boosted Explosion if it absolutely has to. On top of that, with a neutral nature it can still run something like Ice Beam to nail Landorus, Hippowdon, Gliscor, etc. With only a Fire weakness to worry about, he's not hard to fit onto almost any team.