I think the most flexible and overall-best Gengar set (from experience) is Gengar's Pain Split set.
Gengar @ Life Orb
Ability: Levitate
EVs: 252 SpA / 252 Spd / 4 HP
Timid Nature
- Substitute
- Pain Split
- Sludge Wave
- Shadow Ball
This set doesn't have the coverage that sub/protect+3 attacks sets have nor the sniping abilities of Mega Gengar, but it has the potential to come in at many points during a battle and rip stuff to shreds. With a Life Orb, Gengar can basically 2HKO just about every single OU Pokemon that isn't a pink blob or a sandy dinosaur, so there isn't much that actually stands a chance against this thing once it's behind a sub (not that hard to achieve). Pain Split allows you to be near-reckless with it, to the point where most of the time Gengar will be below 50% HP. However, being able to choose between outright attacking and sapping HP is what gives Gengar an awesome edge, and at its heart, it requires no real setup since it can sweep late-game with just Shadow Ball and Sludge Wave. Now that Focus Blast isn't 100% necessary, I've found running Sludge Wave pays off better. It's not hard finding teammates to dispatch Tyranitar and Blissey.
Now that I think about it, the beauty of all Gengar sets stems from their flexibility. This set is no exception.
Gengar @ Life Orb
Ability: Levitate
EVs: 252 SpA / 252 Spd / 4 HP
Timid Nature
- Substitute
- Pain Split
- Sludge Wave
- Shadow Ball
This set doesn't have the coverage that sub/protect+3 attacks sets have nor the sniping abilities of Mega Gengar, but it has the potential to come in at many points during a battle and rip stuff to shreds. With a Life Orb, Gengar can basically 2HKO just about every single OU Pokemon that isn't a pink blob or a sandy dinosaur, so there isn't much that actually stands a chance against this thing once it's behind a sub (not that hard to achieve). Pain Split allows you to be near-reckless with it, to the point where most of the time Gengar will be below 50% HP. However, being able to choose between outright attacking and sapping HP is what gives Gengar an awesome edge, and at its heart, it requires no real setup since it can sweep late-game with just Shadow Ball and Sludge Wave. Now that Focus Blast isn't 100% necessary, I've found running Sludge Wave pays off better. It's not hard finding teammates to dispatch Tyranitar and Blissey.
Now that I think about it, the beauty of all Gengar sets stems from their flexibility. This set is no exception.