I'm wondering, what's the standard moveset for Slaking/Regigigas?
Anyway I'm theorymonning this to be ok:
Gengar@Spooky Plate
Modest nature
Magic Bounce/Prankster
Judgement
Aura Sphere
Tail Glow / Shell Smash
Substitue / Spore
The best thing about this is that Impostor Blissey can't stop it. If it comes in, it's Judgement will be Normal type since it doesn't have a Spooky Plate, thus allowing Gengar to beat it every time. It can also set up against a Choice Band Slaking locked into Extremespeed. If it gets a chance to set up (especially twice), then be prepared for a sweep. The choice between Tail Glow and Shell Smash is simple- either boost faster with Tail Glow, or be able to beat faster threats like Deoxys-S with Shell Smash. The last slot is variable too- Substitute to guarantee some extra time, or Spore to cripple something without Magic Bounce.
What makes Gengar unique is excellent Speed and Special Attack, and an equally excellent typing, making it immune to most of the priority moves in Balanced Hackmons. It's perfect neutral coverage in just two moves is the icing on the cake.
You can choose between Magic Bounce and Prankster. The former is reliable in many situations, mainly for Prankster Sporers, while Prankster can be used with some prediction to Spore a slower Prankster user. However, you must be careful with the second option, since if you use Spore against a Magic Bouncer or attack a Prankster user, then Gengar's as good as dead. Still, in theory, with Prankster Spore, this is theoretically unbeatable, assuming you know the opponent's ability.
Opinions?