Jay, these are some of the forums that are in my "favorites" list. I have listed them from what I think are the most-posted to those with lesser posts.
1. Orchids - I grow a lot of them and there are some real experts that post here regularly
2. All Things Gardening
3. Ask A Question
4. Sandbox - Covers anything and everything. I started a thread titled "Adios, Dave's Garden" that has drawn a lot of interest of those that also visit/subscribe to Dave's Garden or have visited/subscribed to it in the past.
5. Edibles and Preserving
6. Houseplants
7. Plumeria - not so active in the winter months but really gets going in the spring and summer
There are probably many, many more forums with lots of interest and posts, but I have enough to deal with, having 20 forums as my "favorite".