Halo 1 is my favorite but only because of the whole LAN thing. We'd play from 5 PM to 5 AM. All kinds of drama ensued in the meantime. Relationships were broken up, fights almost happened, owning teenage girls through AIM (the whole "Hey, download this MP3" and we would get access to look into her computer), and my friends' MILF mother who would wear tight wife beaters as she went up to give us pop and food. Good times, good times.
Halo 2 just wasn't that exciting. The multiplayer levels I thought were inferior and the notorious ending.
My friend got Halo 3 and despite watching him play, it just seemed like Halo 2 + HD graphics. I still felt that the first one had the best map designs. Bungie made Halo 2 and 3 maps far more complicated and somehow, that was supposed to mean that they evolved.