Multi/coop. Single-player gets stale. I remember playing Bungie's Marathon (quite some time ago) and it was fantastic. There were regular updates, people built custom maps, etc., and occasionally the game really surprised. But then the graphics started to get better, while the "AI" (as everyone calls it) didn't. The last single player campaign I've played was COD Black-Ops, and it was just tedious. They're trying to turn it into a movie, but unfortunately the script never changes. OTOH Black-Ops' multi-player experience is also quite bad (on a mac). Still, MP it is, for me.