I prefer phpBB for the free community and very good coders, i like IPB but it's more expansive to create a small board just to display tips, graphism or other...
I think phpBB is the best for the free source code (but not for template), i hate the style of phpbb, no more good designers....
phpbb is one of the fastest forum software..
software like vbulletin asks more server memory and stuff..
Also you can do lot more with phpbb ( mods and snippets ) way easier then other software