Maybe I'm just biased because a few of them are mine, but I think ZetaBoards has some of the best themes of any software. When I click on a ZB forum's link, even if I've never been to it before, I know there's a pretty good chance of it having a decent theme. I can't say that about many other software choices - especially other free ones.
I would say Xenforo followed by IPB have some of the nicest themes i've ever seen. Mike from Audentio Design does some amazing work for both platforms. 😀
IPB has really great designs, XF ones are good too but I still prefer the IPB ones. You can check a site like IPB Focus, the guy there called Ehren really knows how to tailor an excellent theme.
All paid forum software has really high end designs. I think this is because the designers of those know that most people pay for their license and thus would even pay for a good high end design. while most free forum software's themes are not so high end because most paid designers do not ant to put all that tim and effort into making a design o not make any money from it.
I don't think personally that the forum software is where the good designs are. I think if you want a high end design you either have to make it yourself or you must hire someone to make it for you.
So a couple years after I got out of forums, I built a website for someone else and they wanted Xenforo. Now I have my own again and just decided to keep using Xenforo.
I think IPB is easier to use and customize but Xenforo looks more sleek.