Well ...
I'm running phpBB which seems to be VERY stable.
I used to use the forum that came with phpNuke but it's modules as I soon discovered were buggy - and half the time not working. Eventually - I locked my self out of administrative functions by requesting my admin password.
What's a forum??? There are several "content" management systems out there that do the same types of things. ie: allow users to post live... I think the whole forum issue is blurred now that blogging is da' deal.
I would recommend searching for "content management" and doing some study before making up your mind.