I'm active on the NAWCC message board and WatchUSeek(the latter as a moderator). I'm also active at NAWCC Chapter 149, although there admittedly isn't a lot of activity on the public portions of that board. There is a huge amount of expertise and knowledge at 149, particularly if you are interested in relatively obscure and/or high grade American watches(which I am).
The NAWCC board is, of course, mostly geared toward vintage/antique watches. The most active watch forum is the American Pocket Watches forum-this suits me fine as it's where my interest lies, but I realize I'm part of a relatively small demographic that almost exclusively focuses on this area of collecting. The expertise there in the "major" brands-including Hamilton, Illinois, Elgin, and Waltham(to some extent) is invaluable.
I'm admittedly biased as far as my opinion toward WUS, but I feel like it's probably the best all around watch forum with(active) boards to appeal to almost everyone and an overall great atmosphere.
If you are interested in vintage American wristwatches, another board worth mentioning is the Vintage Watch Forum. Many of the experts in their respective fields(particularly Hamilton, Illinois, and Elgin wristwatches) hang out there.