Let's see.
Vesta - very specific, very targeted at the niche it wanted to target, but support was sporadic, the creator was firmly 'don't customise it unless in the specific ways I tell you you can and don't you DARE share your customisations'. Had a lot going for it in some ways but really rough and unpolished in others. No longer available, and honestly, I don't think many (any?) here would have used it. I am still entertained by the notion that the admin panel is poop-brown by default.
Woltlab - it's very functional. Chooses the path of quiet reliability over flashy features, and solidly gets the job done. Can't speak to the support experience, never used it, but I'm given to understand that for the longest time, German users were prioritised and English users were firmly second class citizens, but since WBB 4 it feels very on par. Also I appreciate the time and energy the devs spent in discussing things at TAZ.
CBACK - it's functional, but definitely got its quirks. Decent performer but you have to fight with the relatively idiosyncratic ACP to get anywhere. Solid enough performer, though not sure I'd be recommending it over even Woltlab (though it is cheaper)
Sngine - a self-hosted Facebook-ish clone with a forum module. Could never get the damn thing to work properly, so I assume it was just busted. The rest of the ACP is a chaotic hellhole so I never bothered trying too hard to fix it.
Now, let's get on to the platforms you've actually heard of.
vBulletin - in its day, it was absolutely the mark of premium. If you weren't on vB back in the day were you even a 'real' forum? (Yes, the free options existed but there was very firmly the feeling that if you weren't on vB you were doing it wrong). And it's certainly fair to point out that vB 3 went for a kitchen sink approach. Which was good for users, and to a point admins, but it sprawled to a point that security was kinda whack-a-mole for a bit because it was almost too flexible in some ways. The buyout from IB did it precisely zero favours: whatever you want to say about the lawsuit or the decision to offshore the development, it almost doesn't matter: the decision to rebuild vB ground up for vB 4 and apparently again for vB 5 is fatal. Rebuilding from scratch is one of the time-honoured bad ideas in software development -
it was just as true 23 years ago as it is now. And without the Jelsoft vibe, there was never any hope for vB to ever recapture what it had been. Mostly now a historical curio than a serious forum platform.
XenForo - what vB 4 should have been. Clearly the devs learned from their mistakes in building out vB 3, and went very firmly the other direction. But that's proving a downside for them: their firm attachment to 'what is a forum' and reticence in producing first party addons to augment that for what site owners clearly need isn't working out for them the way it used to. Maybe 5 years ago I'd have said that XF was *the* forum platform to build on, but these days I'm less sure. If you're building primarily a forum, or something based heavily around a forum, it's not a bad starting point (it still has an across-the-board good experience), but the lack of first party support for things beyond 'just about a forum' is starting to wear thin. If you're at the stage where 'you need several addons out of the box', something has gone wrong. Not to mention the absurd time between chunky releases. That said, if you do need just a forum, XF remains best in class for that, both in terms of admin and user experience.
IPS - I can't fault IPS for the general sense of trying to cater to all the bases, which is something it's really going for, because of the recognition that what their users want isn't just a forum but all the tools to cater to communities, almost regardless of what 'tools' that community needs. Probably the only major omission I see right now is the ability to community-crowdsource a knowledge base but that's a relatively rare use case. The bigger issues I see with IPS are the fact that it's clearly moving out of the hobbyist market (which I can't *fault it for*, they're going where the money is, as any sensible business should) and leaving behind a vacuum that isn't going to be filled, and that in their pursuit for all the things, they're going for breadth rather than depth. For example, as someone who spent years in the e-learning field, I was surprised by the things it didn't support; e-learning is more than just shoving content at a user and asking a sort of vague quiz (especially given how much content is already out there in formats simply not supported, e.g. SCORM, h5p). If you need more than just a forum, IPS is a worthy consideration but it's not cheap for what it offers, depending on your needs.