STAY AWAY FROM PHPBB!!!!!!!!!
It is one of the buggiest and most hacker-prone BBS forums available today. Plus the interface isn't nearly as clean looking as those provided by vBulletin, SMF, etc.
Similarly, I cannot recommend UBBthreads (this forum's successor). We used it for two years and were hacked 4-5 times due to security holes in the software. Additionally, after the first year our subscription with the company expired, and we had to obtain critical security patches via third-party means because we were no longer paying customers. That's a pretty bad business model if you ask me. Similarly, some features just plain didn't work and we were left out in the cold on those also. Additionally, even incremental upgrades in the software were a PAIN IN THE *** to implement.