One more vote for the awesome underground city, though it looked unrealistic, but it is an animated show after all. I can't imagine if such a thing existed in real life, that would be so amazing. They could have done a better job with the demons, but they were ok too.
I like this one a bit. The underground setting is cool, the design of the demon is vcool, and the way the gang catches the villain is a bit different from the usual Scooby-Doo formula. They actually spot him out of costume and chase them, and then have to unmask his human disguise. Pretty neat, I gotta admit.