phpBB is the best. SimpleMachines is the second best.
I think this because phpBB is very user-friendly, has many MODs, has many neat templates, and a new version is about to come out. SimpleMachines forums is user-friendly, and it has a neat MOD installer in the ACP, but the templates really aren't all that good, and the support team isn't as, well, friendly as phpBB. Also, phpBB has been around longer and has better software; they come out with fixes right when a problem is found!

I hope I helped! :)