That's a great question, I am curious myself. Between my webmaster, wedding, travel and wine forums... I can't really pick a fave or most visited - I love them all!
alot of times i go and look for a forum, browse thru the contents and see if it is what i am looking for, and if it is then i decide to go and register. so i am actually part of a ton of forums. some i keep up with more, others not so much. for me its more a matter of time constraints seeing as how i hav school on top of my job on top of my four sites.