I discovered CoinTalk last month and I think it's a great forum, well run and generally friendly. The NGC forum has less traffic but is generally a nice place to visit. But over at PCGS they are constantly "outing" allegedly bad eBayers, giving newbies a hard time and making it a generally unpleasant place to be. Of course, this is just a reflection of the boss, Don Willis.
Cointalk is like the the minor leagues of coin forums, whereas NGC and PCGS are the major leagues. NGC has a far more robust message board system, and is generally more hospitable than PCGS. Yet, PCGS has far more traffic for reasons unknown to me. Moderators are more hands-off on the NGC forums than they are on the PCGS and Cointalk forums.