I don't own a forum yet. Probably one of these days, I would try to monetize a forum once I have already mastered all the little details about managing a forum. Anyway, I have been hearing/reading positive reviews about vBulletin. I think it is one of the most easiest forums to navigate. XenForo is also a good one.
Running IPB4 here, and I love it! I think XenForo would be somewhat cheaper, and not just because it has no renewal costs, but at least I get great support and neat spam prevention with an active license, so I don't mind paying that $50 a year or what it is.
Sadly compared to IPB3 the mod community is still somewhat behind, but luckily almost everything I want to customize can be done without any mods. So my vote goes to IPB!
The only forum script that I have ever used is MyBB. This is free and one of the most used forum softwares. I don't have any experience with other forum software. I have checked many forums that run MyBB and they look great.