The infinity level, ported to the M2 alien textures. It's no longer vacuum, and all the baddies are gone... but so are some of the windows (which makes for easy transfer from hallway to hallway, actually).
Based on the original Arena in M1, but with much wider hallways. (And an extra secret or two...)
A straightforward arena (which, incongruously, has lights hanging in a clear sky), connected to an absurdly overstocked ammo room (so much so that it blinks out of existence as you walk past it-the engine can't draw everything there). Would be more sporting without the ever-present invinicibility...