for me its up in the air between IPB (v 1.3.x) and phpBB...although I have to say IPB has been easier to work with (I can't seem to get all of phpBB's files to upload to my FTP, on this site at least)
I'd go with vB or a newer IPB but I just don't have nor want to spend the cash to buy a licence for it...
(although if there is a free non-pirated version of vB I'd like a link
)