I don't think the question was getting at which forum *software* (of which there are many: phpBB, SMF, MyBB, XenForo, Invision Community, Woltlab, FUDForum, Thredded, AsmBB, ElkArte, Vesta, StoryBB, YAF, YABB if it's still around, Flarum, NodeBB, Discourse, and that's just a selection of the ones you can install on your own server, and before we talk about options like ProBoards, Jcink, Forumotion/Forumactif, CreateAForum, SMF4Free or any of the big boys who will host it for you)i can't think of any, other than XF,... unless maybe they all look alike??
But more... if you go to any given forum, of which there are many, what is a make or break (in particular break) feature.
For me, hitting a forum and being expected to register to see anything is an instant no. Worse, the forums that expect you to reply to a post to see the post - to artificially inflate the post count and make the creator feel good, except it doesn't actually achieve either.
I'll put up with a bad looking forum if it has things that interest me there.