If you want security and functionality (especially from an admin point of view) out of the box go with vBulletin. If you like hacking, and being hacked, go with phpbb
phpBB. Free and I can make it work as I like it. I have a VB too, but the forum I am in love with is still phpBB
Yes, clean code and a thin core. Lots of mods available so you can implement many Vbulletin features easy. The next version(1.3) will feature an extension engine, so you can install/uninstall features easy.
Can't compare Vb with some free version of forum board, If free version my 1st choice is SMF, second goes to mybb