I'd agree with Vaughn, both phpBB and vbulletin will do a good job, but I also would prefer vBulletin if the funding was available. Things like user and group management are more intuitive and the modification system seems to work with less tweaking needed. I've ran a friends site on phpBB for years now and had very little trouble with it, however he moderates everything himself, so group management hasn't been an issue. Which ever you go with I'd recommend using mods/plugins to utilize recaptcha and stopforumspam. Using both literally stopped 99% of the spam-bots overnight.
To a small child, the perfect granddad is unafraid of big dogs and fierce storms but absolutely terrified of the word “boo.” – Robert Brault