I like a light one because a have a small pack of visitors, and I don't need whistles & bells that big forum application has (ex: private message, avatar, etc).
Second, cmiiw, if register globals off we can't past any variable without super globals didn't we? which are most light forum software are don't have (because there are no light forum apps as new as PHP 4.2, when super globals introduced). Please correct me if I'm wrong because I'm new at this.