It depends how you want to use your Haunter. If you end up using the Jolly nature, I would recommend using a physical set or a Curse set as the Jolly nature lowers your special attack but raises your speed. But using a Timid or Modest nature is more recommended as the Gastly line has a beastly special attack which actually makes it viable for competitive play
Tl;dr: use a physical set if you keep Jolly, use the much better special set if you go Timid(or Modest if you get that one instead)
