Beaver creek, blood gulch, pretty much every CE map
I would say this too, but Halo 1 maps were designed to be played with fall damage. Playing them in Reach's Anniversary Classic playlist is a totally different experience than in Halo 1, though it is still fun. Damnation, Hang 'Em High, and Prisoner were all much better with fall damage, though.
Halo 1 LAN vid: http://www.youtube.com/watch?v=VhjWnJU2EKY