I love Scarf Gengar, I've used it extensively so I can vouch for it. I think that Destiny Bond and Sludge Bomb deserve AC mentions, as the former practically guarantees a KO on stuff like Pursuit users, or simply really bulky/powerful stuff that Gengar can't outright KO. Sludge Bomb deserves a mention because it's Gengar's most powerful special move, and can nab kills on stuff like Breloom. Shadow Ball's better super effective coverage does obviously make it a better STAB move most of the time.
▄█▀ █▄ █▄█ ▀█▀ █▄ █▄█ ▄█▀ ▀█▀