Invision hands down. I find it to be extremely secure; I have never seen any up-to-date IPB powered forum attacked, not even by spam bots. SMF on the other hand, I've seen that spammed, hacked, sessions hijacked, and all sorts of other things.
vBulletin is too slow for me, and WotLab is basically a POS compared to most forums. Also, IPB has an extensive amount of addons, modifications, skins, etc. I daresay it is even comparable to phpBB in that respect.
But I don't use IPB for most things, only on projects that I think may become popular, need the extra security, etc. On most of my sites, I use SMF or phpBB, depending on which one has the themes and mods I need.
On choosing from only paid, I'd have to say vBulletin; however, I would also jump on the OS bandwagon on this one. From my playing, I've found SMF to be the best overall so far for ease of use and customization. I definitely recommend people to go that way if they have a choice to use an OS one.
Every one of the choices in this poll is Open Source software. Free as in free speech, however,
not free as in free beer. It is possible to do that
EDITED BY akitchin: i don't think people like having to copy/paste text into another program just to read it.