4 HP/252 SpAtt/252 Spe
This set has a lot of surprise value on an offensive team and can do its job surprisingly well. The role of Gengar is really a sweeper in this sense, but it can be used as a one time sacrifice to remove your team of powerful HO sweepers to clear the way for yours. This set bluffs LO Gengar sets (SubSplit and Sub + 3 attacks), dragging in things that would RK it, such as TTar, Scizor, and pretty much any powerful sweeper in general. with a Timid 110 Spe, it pretty much outspeeds every single boosting sweeper in OU (and speed ties with Latios). Simply put, it can stop SD and CB Terrakion, SD and CB Haxorus, SD Virizion, Infernape, Tyranitar, SD and CB Landorus, non-scarf Rotom-W, and much more. It can also make speed-boosting sweepers a little hesitant towards attacking, such as DD Dragonite, RP Terrakion, and other stuff.
You could also theoretically run a Scarf set to counter all of the Speed boosters out there, but I feel it really hinders Gengar's ability as a lure and killer.