Well, I haven't used SMF yet, but I've registered on a forum using that engine; it's pretty nice, actually. A lot better than ProBoards.
PHPBB2 is as easily spammed as deerkill. I've seen two worthy forums using this software being plummeted by spam (coming up with some weird Finnish (something like that) names).
PHPBB3, however, is a huge improvement, but still not quite as good as an SMF forum. It misses some of the crucial UBBC buttons (custom UBBC buttons sometimes don't work), and it's not very customizable. However, admin panel looks nice, and everything's straightforward.