phpbb is out of the question for me, other than vbulletin, I used MyBB until I was finally able to purchase a VB license. MyBB is user friendly, plugins are a snap to install, and template modification is simple. IPB, I never gave them a second look after the IPB 2.0.0 series but I do participate in an IPB forum and from what I can tell, their trying to play catchup with VBulletin.
PHPBB3.0.6 for the win. better then SMF better then MyBB!
I have never used IPB but....
Phpbb is not as good as VB but it is free and has no problems running any forums that I have used so I would not recommend upgrading unless there is some special reason to do so.
MyBB ftw. it's all i've ever used and it's all i ever will use. 1.4 > SMF2 > phpBB3. phpBB is a pain in the ass to mod, and smf requires safe_mode to be off to mod, MyBB requires none of those, as it's based on a hook system.
Phpbb, definately. Its got most, if not all of the feature of vb, but in different ways, and is also free. I love its interface and just find it generally easy to use.