vBulletin - and it's not free, but it is feature packed, solid performance under heavy load, stable releases not too frequent, very secure, follows DOM standards and is accessibility compatible, plus it has a template plugin and hook system which is the most powerful out there so the core code does not have to be hacked in any way. The themes are also very elaborate with properly used Ajax and the technical support forum is superb. I'll be honest, nothing else even comes close - this is by far the best for the reasons I stated. The standard disclaimer is not everyone needs these features, nor does everyone need to customize using the powerful hooks/plugins system, and of course phpBB has been around so long people want that basic layout as it's most familiar to them.
But when you start looking at performace and features overall, vB rocks. If you want free, and that's all you care about, stick with phpBB as it is the most robust, easily hacked, best supported and simple to use forum out there from the user's perspective.
And, of course, it's used here on macrumors - but I am a licensed vB user on my other forums so I'm not saying this simply because it happens to be used here.
-jim