phpBB is getting very old now, but it seems to be the only one left worth using for dedicated heavy duty forums.

bbPress serves a much needed purpose and is the only one I know of which adequately fills that niche. SimplePress is not the way to go IMO.

If I were setting up a dedicated social community, centered around a forum, I'd probably use phpBB due to all the extra features. All my forums are currently on bbPress though, as I don't have any heavy duty forums, just simple question and answer type sites. I also hear good things about the performance of phpBB vs bbPress, which doesn't surprise me since WordPress has quite a few problems in that area.