The one with the most members that I’ve found so far is the forums at FatWallet.com, with somewhere like 58,000 members.
I don’t think there is a good answer to that. Many message boards don’t require registration, and some keep inactive accounts open longer than others so it’s hard to tell how many active members there are. There are also companies that provide message board services to various groups, like ezBoard and Yahoo! Groups. Do you count the whole system as a message board? ezBoard, for example, claims 10 million members.
If you count entire systems, I think the largest is Usenet.
From what I understand of what you’re wanting… boards such as the ones over at somethingawful.com and arstechnica.com are going to be the biggest on the net.
I would say Fark.com
IGN boards at ign.com has 754,542 members most active boards i have ever been to. sometimes i think this board moves way to slow when it takes more than a few seconds to get a response.
Ezboards IIRC will require you to register for each Ezboard you use which would bring their member numbers up. Also they still count those people who register a million names and only use one. It’ll be very very hard to get an accurate number.