Ikonboard gets my vote, but then the only other ones I've tried are YaBB (bit of a pain to admin, slightly clumsy interface) and Invision (very easy to use, but I simply prefer the Ikonboard UI again).
All IMHO of course!
Also having used Ikonboard so much I can now customise it very easily since I'm familiar with the code! (Although it did take hours the first time!)
James