Gengar is one of the most versatile pokemon, so if i were you i would build it around what the rest of my team lacks.
best nature is probably timid followed by modest
lots and lots of moves to choose from:
damaging moves like shadowball, sludge bomb, giga drain, thunderbolt, focus blast
status moves like substitute, disable, willowisp, toxic, protect, even perish song
i know this is vague but i hope it helps