My guess would be the Bioware Social Network. But that is currently just a guess. There way a rather large community on the old official Bio Boards before they shut down. unfortunitly the Forced move to the Bioware Social Network seems to have fractured the community. It would not supprise me if some of the PW boards have more activity then the official Bioware Social boards.
The NeverWinter Citadel Project may even surppass it with NWN1 members, even with the needless bickering between NWN1 and NWN2 in the past.
EDIT: my Biggest problem with The NeverWinter Citadel Project Is that is is hard to tell what can be used for just NWN1 over NWN2. Even though they say and invite NWN1, The boards are mainly set up for NWN2.
EDIT2:
http://www.harvestmoonconsortium.com/ is anither place I can see growing into the future.