Toxic to badly poison the foe and lower HP quicker, Mean look so they can't escape once poisoned. Shadow Ball just because Gengar really needs an attacking move and Destiny Bond to give a nasty shock to your opponent.
I always recommend Gengar to people. Myself, I would get Gengar but I refuse to change my router security to WEP so I can't trade. Instead I gotta save money up for the WiFi USB Connector.
That's a good move set but not recommended unless your going online. In gameplay that would annoy you more than they annoy you.
Now I'm gonna give out 2 move-sets I would use. 1st is being cruel and 2nd is being more offensive.
Hypnosis (to force Pokemon into a nice little map)
Nightmare (to make sure the Pokemon never wants to sleep again)
Dream Eater (to destroy it's dreams and the Pokemon itself)
Shadow Ball (to destroy the Pokemon)
Dark Pulse (I think it can learn it)