Best Forum Software | MyBB, Xenforo, IPB?

What're important to you? Is it free price tag, or abundance of themes, or ease of migration from another platform?

Personally, I prefer IPB the best, but that's me. It's because I have been using InvisionFree in the mid 2000s through early 2010s, and that's a free SaaS service based on an early version of IPB. If you go with IPB, you need to go with version 4.x and above. Otherwise you need PHP 5.x for version 3.x cos the 3.x version calls some legacy PHP APIs that have been deprecated for long.