Tom Clancy's Rainbow Six: Vegas

Oh my god I played SO much of this game. The multiplayer was fucking awesome. Not just the fact that you could do coop trying to kill the bots and clear out levels together which was SUPER hard but because it was one of the first shooters online that an XP system.

I spent so much time doing deatchmatches and coop missions trying to grind to get better and better gear.

Oh my god I forgot not only did they have XP and ranks but they ALSO had customizeable gear. This was before Call of Duty ripped it off. It was a hugely original feature. I remember once you got high enough in ranks you could get really cool custom camo patterns. I remember seeing a max ranked dude who had a fucking confetti pattern on his gear. I wanted it so bad but it would have taken SO long to get. I was pretty good and got pretty far (I'm REALLY curious to know what rank and stuff even though the servers are prolly gone). What a great time it was.