I love co-op shooters- playing straight through Quake and the two extra mission packs with three friends was absolutely fantastic fun. The only other PC game that has come close is probably Serious Sam - the only game where you got to play any one of a selection of gay disco icons and shoot giant space bulls in ancient egypt - pretty boring as a single player game but get a few friends along and it completely and utterly kicks ass.
They don’t seem to make them like that any more, unfortunately…