I'm probably alone in this, but I like
bbPress personally. It's extremely basic, but that is the great thing -- it's not bloated, and you can customise 'til the cows come home. You can always tell when a site is VBulletin powered, because it's not very easy to customise much, I believe. I am sick to death of forums which sport their software's default theme. bbPress is a framework, and lets you get it exactly how you want it. There aren't tons of features as standard -- but the idea is that you download plugins, or even make your own. (And it's
completely free and open-source; the police won't arrest you. And if you have a WordPress blog you can integrate it with your bbPress forum. And bbPress has
Gravatar support. Yeah!)