The Halo series is win. If you havent played the whole thing, dont criticize it. I didnt care much for it either the first time i played it. you gotta give games a chance. Halo2 was overrated, yes, but there's a reason I still play the halo series, and its not because I'm a fanboy. Its just fun. I've been playing the first halo game for almost 4 years now, and the campaign is still crazy fun for me. Nothing beats coop campaign. The campaign is a heck of a lot funner than most other games I've played, including HL2.
If I had a 360 I'd have Halo 3, and I'd be playing that nonstop.