Recomendations for a forum plug-in?

I'm working on a project that needs to plug-in a forum for users...

In researching what to use, I have come across: Beat, R Form, F Form,

Generally speaking, the requirements are for a package that be
configured to work at this level of sophistication

(what I am calling) threads -> topics -> comments. Highly configurable
(to turn off features we dont need). Highly skinable. Secure (against
trolls, spam bots). Support private messaging between users. free and
open source.