Which forum software is the best??

I've used a lot of forum software and xenForo imo has the most addons and probably styles (at least paid ones) - personally I really like IPS4 because it's very user friendly to use, active community and the support team is fast and responsive. But at the end of the day it's not about what others think, it's your forum and I would suggest maybe try all of the major ones yourself (if i'm not mistaken they all have a free demo for you to try out) and weigh the pros and cons of each.