In my opinion PHPBB. You can find all the support online you need. It's a common choice, which makes it easy to work with.
My current forum, Admin Juice, is hosted with the MyBB forum software and let me tell you, it is very good! It is lightweight, and it's addons (plugins) are very simple and easy to install rather than the phpBB addons which are much harder to install.
phpBB definitely. I've used both and had a better experience with phpBB and my users reported the same.