A Day at School was played a lot back in the day, but now that maps pretty much dead. I think I saw some version which had been updated a lot a couple weeks back, but the author had "failed" to mention who had originially made the map.
Bounds themselves were fad maps. Maybe some defense maps following sunken D 10 way as well, since they just basically switched those out for goons or rines.
And some Open RPGs were fun, especially Quests.
yea i remember spending hours on an open rpg just trying to get kills a minerals to buy a town

and hours on a special forces map

I'd say Grand Theft Auto. I haven't seen that one on b.net for a long time!
The original Turret Defense was pretty popular for a while.. probably cause it was rather hard but possible.
I don't care much for anti-air D's. As far as I'm concerned, in a true defense game, the whole point is making mazes that are, although possible to get out of, give the computer traffic jams, turning around and around in circles, being stupid, and generally never noticing that one allyway to the goall... >=D