Xenforo or IPB

since you have asked only about two, i will point them out here

IPB is easy, not easy to code for it but easy to use, everything is like with a click, there are lots of themes for it especially for older versions as i have been using it for 2 and a half year i guess, its not fast as xenforo and also not fast as mybb

Xenforo is not that easy but still a lot easier than mybb, themes for it are awesome, lots of customization's but not many as IPB has

inshort, IPB is popular because of its stability and even getting better day by day but still not fast as Xenforo and MyBB, that's the big problem....everybody wants speed first but its fast if you have a dedicated VPS, LOL

Choice is yours!