Ack, my three nominations disappeared. Here we go again:

http://www.ayyas.com (lots of man hours had to have gone into that one; the idea doing a vB theme which eliminated all tables always appealed to me).

http://www.sherdog.net/forums/ (for you MMA fans...and the community attracts the attention of some of the biggest names in the business).

http://htmlforums.com (I've always liked the look of this one...it was a big inspiration to me back when I first started designing vB themes).

There again. :)