some great recommendations,
lets narrow it down a little,
Would be good to have 2 very good co-op FPS's, we're about 5-6 players so it'd need to support that much either going through the campaign or scenarios.
SWAT4 looks great, as does Raven Shield even though its a little old now but I've heard loads of good things about it.
Is AvP 2 co-op? The first one was where you could both hold up in a bunker with endless Aliens coming, that was pretty awesome but I can't remember if the 2nd was.
Are there any fun sports games out there? In the same way Flatout2 is awesome fun and not too serious, a sports game that is the same would be excellent.
RPG I'm a little stuck still, how is Neverwinter Nights 2? Allow 5-6 player co-op? It's a little 'deep' isn't it? Rather than Diablo-esque.