My personal favorite seems to be Invision Power Board, but it's not free: http://www.invisionpower.com/
Second fav, VBulletin also not free: http://www.vbulletin.com/
Third, SMP, but only because it doesn't seem to have the 'skip to newest thread' feature, but *is* free: http://www.simplemachines.org/
I wasn't able to find /any/ converter from UltraBoard 1.62, definately post the converter if/when one is made, seems to be requested alot, but never full-filled.
I admin a SMP based board, I'll see if one of our PHP guys could help you out.