phpBB isn't too bad, provided you keep up with the security updates - I've used it without too many problems on a number of forums over the years. vBulletin is probably the most "professional" forum system that I've come across, but given how many web sites I run the licence fees make it too expensive.
Hmm... show me a vBulletin board as SEO friendly, fast, and good looking as this one phpBB with the right touch = teh win