Halo 1 > Halo 2
Why? Because to me multiplayer doesn't make a game. The Campaign in 1 is just great, and the weapons and flow of Halo 1 makes the game a challenge. Co-op in 1 is probably the greatest halo experience i ever had (beating the game legendary co-op, multiple times) The weapons in Halo 1 are well balanced, compared to Halo 2.
Yes the multiplayer in Halo 2 is great, but like i said, i'm a single player. The levels weren't great, though the new features did help keep me playing. The storyline was terrible however, and that threw me off greatly. Dual Wielding was the best idea they came up with truly, but the weapons are worse now than before.
Here Comes A Special Boy!