Sweeps, I love me Halo 2, I really do. I just, for some reason, have a hard time ranking it above the others. I'm not sure why, really, because that game has probably the most impacting campaign of them all (aside from, you know, ending the war in 3).
I think I have it at the bottom because, of all of the games, its the one that I played the least. Still, I absolutely love going back and playing it, and it grows on me every single time.
Also, on the topic of Halo 2, bring back Arbiter missions!