Out of the paid ones probably Invision Power Board. I'm not sure why exactly but I just prefer it over vBulletin.
Out of the free ones, MyBB all the way. It has features very close to vBulletin and many people consider it the free alternative to it.The latest version of phpBB comes a close second to me. The only problem with it is that it's admin CP is quite cluttered and hard to use.
vBulletin, if you can cough up USD $150. Otherwise, you can't go past SMF, it's fast, safe, reliable and doesn't require a bazillion (as yet non-existent) extensions to get the most out of it (*cough*phpBB3*cough*).