VB - never been an admin/ mod on it so I cant comment
Ikonboard is a pile of ******* I despise it
XMB is owned by a friend of mine, and I know that it has gotten alot better (old versions didnt have encrypted databases.etc).
IB - very cool board
phpBB - the one I use the most and hasnt let me down yet. (phpBB2 is a bit bloated though).