The best for me was Black Hawk Down, online. Up to about 64 players, the sequel had 128 players, but that was fantastic. The maps were actually endless, although that was often flat desert after a while, but snipers could actually run up to mountains on the side of the maps and snipe away at the trough in the middle. Wasn't completely unbalanced though, as upon respawning you could head up to the mountains and hunt them down.
Fantastic game. Mag looks far too hectic to be much fun for long though.