best one is woltlab burning board
http://www.woltlab.com/index_en.phpI host 4 wbb forums and didn't see a single bot enter so far, my smf forums got attacked by bots but easy solved ...
Put a captcha at the submit new topic/reply for lets say 1st post/new member. xrummer the spam bot engine isn't so smart yet to check for a captcha at that location. So the SMF spam bot issue got solved.
vbulletin, ipb, phpbb2, smf, ... all are vulnerable to xrummer it slices through the captchas like butter
About wbb, big names uses them crysis, motorola, starbucks, bmw, Siemens, MSI technology etc ... so there must be a reason why they choose for wbb as those names have certainly the knowledge in house to develop a forum engine by themself.
wbb has also a full blown framework that is easy to use for your script/website endeavours
