XenForo. I just prefer the look and feel of XenForo. IPB is good but I feel that recently it has been a bit behind XenForo in terms of development. And the pricing differences between the two is just crazy. The fact that the basic price is just for the IPB functionaiity is just weird because you are basically paying $100+ for a software that will only allow people to register and login. You then have to pay for the "modules" seperately. The fact that a forum software sells its "forum" as a seperate module is just weird. It just doesn't make sense at all and I don't see why they would do it. XenForo has a great collection of free and premium themes and they have some great plugins too so that's why I would choose XF as the best paid forum software.
My personal preference in case of the forum software is VBulletin and IPB Forum. There are lot of Xenforo users. But I am not finding much use for my solutions with them. I find IPB and VBulletin to have more features and lot better licensse and solutions. You can have lot better community around those set of the solutions. I guess we have to experiment and check out what may or may not work for each cases. That's what I have learned.
As with the free stuff, I think a lot of the preferences could be more subjective than anything. For instance, the complaints are normally shallow and people simply like stuff with familiar themes they enjoy. Note, in my case, it seems true as I haven't encountered software that didn't seem bad on occasions due to negligence on the part of the forum owner. I mean, even on a XenForo forum I had, there were major problems with profile spammers that probably moderators couldn't do anything about; it had to do with, ironically, a weak captcha from XenForo.
I think Xenforo and VBForum are two good options. But many people are choosing Xenforo no idea what could be the reason. I'd prefer the IPB and the Xenforo for the same. I have found out that these three options are good in many ways. It's just some of the time makes difference depending on the type of activity you may want to do with it.
I have only experience with open source forum scripts. I think as long as forum scripts like MyBB and PhBB are available, there is no need to buy expensive forum scripts. However, if you want to get a premium scripts, I think Xenforo is good. It is cheap compared to other forum scripts. I think you can gte if for around $140.
I have not checked Woltlab as of yet. My personal favorite is IPB. The reason being it has everything and better design out of the box. That's something I don't think Xenforo and others have managed to catch up on. And price is also not much expensive on the IPB side. So my vote goes for the IPB in that case.