i'm pretty much a newbie and what i say should not be taken as gospel. that being said, it is my opinion that phpbb is one of the best out there (if not the best) - the price is, oh, FREE, and it is available at www.phpbb.com
- installation isn't too bad - where you run into trouble is modifying it - though stock it is pretty darn powerful and robust - it is the backbone of able2know, my little board, and a slew of others.
there is also vbulletin that i never really checked out, and some others. in my opinion, phpbb is your best bet. hope this helped, and i'd be interested to learn what you decide on.