I never saw the point of GD or the CB. They're just... a whole lot of nothing.
ED is a similarly pointless pile of dross, except that its users are apparently trying (often much, much too hard-) to pretend that it's OMG!SOMETHING instead of just accepting its nothingness and embracing it the way the CB and GD do. I'll read all three once in awhile just to see what people are up to, but I almost never post in any of the threads I read.
Similarly, I lurk in the AT and the Exchange. They're sometimes interesting reads for one reason or another, but damned do I dislike some of the "regulars" and the way they do things. I don't think I've ever had the inclination to post in either place.
So, that leaves the GCD, SF and Heaven... The only forums I actually post in. I've been in the GCD for quite awhile, so it seems the most comfortable to me. SF has become a rage pit no matter what side of an issue you're on, so I'm beginning to lurk more than I participate there, and Heaven? Heaven is just slow, and honestly no more interesting than the CB. It's just missing the drive-by porn and troll contingent.