It depends on the features you'd like.

I think right now, myBB has the most plugins. That is what this forum runs on.

They all have around the same number of plugins and extensions, but MyBB holds a little bit more.

It's also become very popular. I know vBulletin is lacking a lot because people don't want to pay. If you pirate it, they find out - they can have the hoster remove the forum/account. I believe niftyhost should not allow nulled forum scripts, to avoid any issues with forum developers.

phpBB - I haven't really run into lately. Mostly it's MyBB forums.

You just have to test. There are several websites where you can test MANY open source applications!