Well, since I only download the best maps, I'll just list one of my all time favorites. Act 1: The Sewers, which was created by rifte gifle, this map is really amazing. . . to say the least. But it was one of the coolest maps I've ever played, just get a large party and play on Infection and it's crazy.
I saw the video for it, but it looked really open. However, that's simply the impression that I got. I've played his maps before and I know that they're pretty good. Anyway, it depends on what you're looking for. My favorite mini-game is Extreme Dodgeball. It's incredibly fun. Otherwise, I suggests checking the community maps at least once a day to see the newest maps.