MyBB is my favorite. Easy to set up, easy to use and looks good. Not a fan of SMF or phpBB as far as free forums go.
If money is not an option then I am using vBulletin, however I just did an install of phpbb3 and will say it has come along way since phpbb2 that I first used years ago. The real question is do you think people that visit forums care what software it runs? Will a site be more likely to retain members if it uses a vBulletin as opposed to phpbb or smf?
Having tried phpBB, I will not suggest going for it. It is not very flexible. I will have to enter into "coding" to do some custom changes. Next time I will invest some money on vBulletin, even if it means risking $75 or so.
PHPBB is the best in that list. However i love Vbulliten. Its easy to use, easy to install mods and secure. Try having a loook at that - but its costly.
if you want to go for a free forum version you should probably go for SMF. it's really good also, if you've got a joomla based website, you could try Fireboard. else, i really really like Invision and vBulletin, but those cost money.
if you know php and like to code then go with smf. but if you are like me and prefer to just have plugins, go with phpbb
Out of the two you listed, SMF in terms of security, although I cannot say I have used phpBB3 as of yet, so I couldn't give you the positive there. But from what I have heard SMF. Although if you have a budget and you can afford vBulletin I would definitely say go for it. In terms of SEO, security, customisation, functionality, build quality, etc - I would say vBulletin is 100% worth the price. IPB is another option if you would like something different, although I don't think it has as good of a reputation as vBulletin yet, but from some of the threads I have read in here it could make a swift approach in the forum world.