Battleground
This has to be one of the largest, most complex net levels I have seen. It is really not a net level, although there are many starting points. I believe that the author has put into this level just about everything you could think of. There are some bugs, a couple of untextered sides, some polys not being assigned "alien impassable", but when you download this thing, you will play forever. Some very good visual ideas, but, almost "overkill" in some ways. This author deserves some email, download it, give it a try and let him know.
Levels in map "Battleground":
Battleground