I've never heard of the ones that are considered "bad" that have been discussed thus far in this topic so I am just gonna be the one to put this out there! SMF is gotta be the worst, their team seems to be taking for ever to get out of beta for their latest release. I think they've been working on it for over 2 years now O_O
I don't think any of the forum software are worse - but I won't ever go with PHPBB ever again, it was insecure before when I used it.
phpBB and Proboards. phpBB has so many hidden exploits that are a **** to patch. It just isn't worth it.