DM-Rankin, by far, the most played map of UT2004. It helps a bit that both maps were available since the first demo.
ONS-Torlan, also from 2004, which also received an official remake for Unreal Tournament III. And in 5 different flavours, to top it all. It also has the distinction of being the most played Onslaught map of the game. And for good reasons.