Let's see...uh-oh. I have too many favorites! Hugebutt, all the H2 maps (as I don't own an XBox [yet], it's the closest I can get to Halo 2 without having to get Halo 2 Vista), and CMT_SnowGrove. Though i think that SnowGrove needs energy Swords. There's a server I go to on CE occasionally that hosts infection games on SnowGrove, but it's kind of frustrating because the lack of Energy Swords forces the zombies (blue team) to resort to simply meleeing, which gets infuriatingly annoying, as it gets difficult to catch up to your quarry.
phew...I said a mouthful.