It depends on the set. The SubWisp set fits well on any team that has troubles with stall teams, as it is able to annoy and wear them down quite a bit, as well as set up a Sub in front of many stall Pokemon such as Clefable, Mega Venusaur, and Chansey. Taunt prevents healing, WoW passively harasses opponents alongside hazards damage from switching, and Shadow Ball handles Fire-types that are immune to WoW and provides Gengar with a faster way to kill defensive Pokemon, and also serves as a useful tool for revenge killing offensive Pokemon.
The all out attacking set with Life Orb that sometimes uses Destiny Bond is a great hard hitter that can revenge kill a great deal of offensive Pokemon while being almost impossible to counter with SR up, serving as an excellent softener for late-game sweepers. For this reason, it fits pretty well on offensive teams that keep up a fast pace, can't afford to give set up chances to anything, and appreciate Gengar's mid-game softening skills as they usually pack multiple late-game sweepers. Gengar can also revenge a lot of offensive Pokemon for offensive teams whith its great Speed tier, Speed, and coverage, such as Landorus, Garchomp, Keldeo, and Terrakion.
Here are the sets i am talking about btw:
move 1: Substitute
move 2: Shadow Ball
move 3: Will-O-Wisp
move 4: Taunt / Sludge Wave / Disable
ability: Levitate
item: Black Sludge
evs: 28 HP / 148 SAtk / 80 SDef / 252 Spd
nature: Timid
move 1: Shadow Ball
move 2: Sludge Wave
move 3: Focus Blast / Thunderbolt
move 4: Hidden Power Ice / Destiny Bond
ability: Levitate
item: Life Orb
evs: 4 HP / 252 SpA / 252 Spe
nature: Timid
I really like Hidden Power Ice on LO Gengar btw, as it allows it to OHKO dangerous offensive Pokemon that Gengar can check, such as Garchomp, Landorus, Landorus-T, and Dragonite.