I think it really depends on what YOU (the admin) find to be the best.
Personally, I lean toward XenForo more than any of them, but that's mainly because I'm used to it. I think the other paid options, like Invision and maybe even vb6, can compete, but it depends on what the end-user is looking for in a platform.
As
@AWS mentioned, free platforms might even be able to compete. I know plenty of people who swear by phpBB or SMF and would never go with XenForo.
That might sound strange to you and me, but we're used to XF, and they're used to the platform they use.
To me, this is like the "iPhone vs. Android" argument.
In all honesty, you can make free software do everything a paid software can do probably by spending the amount of a paid software cost of premium add-ons. Just some food for thought, haha!