I haven't messed with Vbulletin so I don't know how good it is...
Phpbb is extremely simple to use and I like that, although I think there's always going to be serious vulnerabilities in that software...There are some gorgeous skins for Phpbb though. *drool*
IPB isn't bad. I like it's features and design. I enjoyed working with it
Plenty of nice skins and overall good software. I prefer it over Phpbb.
I'm using SMF at the moment. Incredibly easy to set up, as well as having nice features. Although it has a serious lack of skins compared to every other software...although it's FREE! Gotta love free stuff right? Can be a little complex to use sometimes, but well worth it.