I'm not keen on VanillaForums, Discourse, Flarum, NodeB and I know IPB can be resource heavy, plus I'm not 100% on the look of that either.
For me, it was between vBulletin 5 and XenForo, and to be totally unbiased here, I mainly choose vBulletin 5 because I already had a vBulletin 4 licence and it worked out cheaper for me to get the best software with as many features as possible and at the best value. Otherwise, both vBulletin 5 and XenForo are much better compared to the rest, IMO. I'm more familiar with vBulletin than the other forums too, and so that was another reason I choose vBulletin 5.
I've used XenForo as a user but not admin so not sure what the administration is like regarding ease of use, etc.
I hadn't used vBulletin 4/5 for a few years, and when first set it up it took some time getting used to it, to say the least. There are loads of minor bugs they still need to fix (can view them all in their bug tracker), but the software does what it's supposed to do. But then what open-source/commercial software doesn't have any bugs? They all have them.
I won't link to the forum I setup to give you an idea what I did with vB5 as per rules, but I'd suggest anyone interested in vB5 or XenForo to signup to the community at both vBulletin and XenForo to see which interface you like best.
Both vBulletin 5 and XenForo have their pros & cons. What one offers the other doesn't and vice-verser.
I'm happy enough to use vB5 for the amount I paid for the upgrade. If I'm not happy with vBulletin later on, then I could just migrate from VB5 to XenForo, but I think vB5 will do what I want it to do which is to have a discussion forum that loads fast (works with PHP 7.1), has SEF URLs plus has a friendly interface for users.
One thing I don't like about vBulletin 5 is I find the Theme Color Builder is not that good, and it isn't easy to make a custom theme with it. That aside, I think the rest makes up for that, but I'd like to see vBulletin improve the theme builder (without us having to edit custom .css files, etc. manually).
I think HD would be best on vBulletin 5 or XenForo, or IPB if you think that is the way to go. Site Owner ultimately needs to be happy with software choice first, users second. You need to manage the forum using interface you like, etc. If you like it, then should have members who will like it too.
In short, I'm not 100% happy with any forum software and wish I was an excellent coder and had the time so I could make my own forum software, but that isn't going to happen, so will settle for vBulletin or XenForo.
PS vBulletin has improved a fair bit since this thread was created (supports PHP 7.1 now which is important in terms of speed, plus SSL, too).
GGood luck with whatever you decide.