Vanilla is simplistic, easy to use.
SMF is cram packed with features, a lot that many would consider bloat.
PHPBB is easy to use but requires many 'hacks' and things like the search facilities are far from perfect. But it works and is a nice balance of features vs ease of use, and is probably the closest thing we have to vBulletin that doesn't cost.
PunBB is up and coming, though I don't know much about it.