Discourse and NodeBB are the only competitors in my opinion. Each has their own draw.
Discourse is the most mature and has the most features. However, you pay for it with rather complicated and expensive setup. It is very resource intensive for a forum software and complicated to alter.
NodeBB I don't know a lot about it but it seems to be relatively mature but I'm not a fan of the design decisions of its developers and it seems a little cluttered.
Flarum is the easier and most cheap hosting and resource wise. It has probably the least number of features but it is the newest and the most promising.