vBulletin all the way. Its the most professional and I think its easier to use/customize than other forum software. And its probably more secure that most other software.
MyBB.
I like it because it is really easy to use. Everything is organized so well. It also has features that phpbb and smf don't have that I think they need. I have learned so much from it by looking at it's code also. I can't wait for the new version!
I like SMF because I use it!!! I don't think vBulletin is bad either. Come to think of it, it is better than SMF. But I can't afford to pay for a forum. Just have to manage with whatever is opensource.
mine, of course, is IPB, as it's easy to configure, very intuitive, nice look, customizable... i tried many software, but could not been able to work with them, as easy, clean, and fast as IPB
vBulletin is amazing but that's really only because it's paid software. If we only had the choice of free software then it would be PHPbb 3 all the way. I don't see what the point in adding the PHPbb 2 option there when it is older and has less features compared to version 3.
I voted for other, i realy like the Futaba software, more open for users to use as no registration is requrired, people can just go on and post anonymously