^Well, personally, I reckon Halo 2 and 3 play a lot better than the first one. The campaign modes and story aren't quite as great but the multiplayer facets make it that much better. Especially now in Halo 3 with the new features they stuck on it. So many more options to pick from outside of 'shooting shit up'. Hell, I rarely play Halo properly anymore, i'm more interested in seeing all the other things I can do with it.
You can get me talking about this stuff all day, i'm a sucker for the Halo games.