SMF is pretty horrible in my opinion... well, it is what it says it is: Simple, basic and customizable. It's pretty lously imo, the layout and Frontend looks horrible, and it lacks any great features. The Backend is okay though... yeah, not a fan of SMF.
phpBB is the second-best choice in my opinion. It's secure, used by a lot of huge sites, and has a nice Frontend. I really like phpBB's Frontend, but it's Backend is horrible.
MyBB is by far the best choice if you want to go free. It looks nice, there are loads of Themes and Mods (and even more if you pay for Labrocca's mods) for it, it's secure (as long as you keep updated) and it really is solid software.
IPB is great software. It really is nice and I love it. Everything great to say about it, but I don't like some of the extra features included. It's still only second to vBulletin though.
vBulletin, in my opinion, is the best forum software available if you're willing to pay. A lot of forums I go on use vBulletin and I love both it's Frontend and Backend (though I prefer MyBB's Backend...). Brilliant software.