Hmmm, let's just get the obvious one out of the way...
THUNDER WAVE / BODY SLAM (Commence with Para-flinch and opponents hating you! Body Slam is a little less favorable, as Gengar has high countering potential. Thunder Wave is also blocked by Ground-types and deals no damage but it's still a mater of preference)
IRON HEAD (Flinch chance!)
ZEN HEADBUTT (More flinch Chance!)
WATER PULSE (Nice chance of confusion; coverage against Fire and Ground-types)
Your basic Serene Grace Abuse set. Slow down faster opposition with Thunder Wave, then proceed with annoying and damaging at the same time! Don't think this requires much explanation; just be wary of people rage-quitting.