Simple Press is probably the most popular. It's robust, but you can tell it is forced in there and doesn't always mesh with some themes. If you use this, make sure you can make some template changes if necessary to get it to look right.
Vanilla forums is probably the easiest, (it's literally one line of code to implement). It, again, looks little forced in there. But otherwise, it's pretty easy to integrate.
BBpress has built in integration with WordPress, so it would probably be a little more seamless. It also isn't the most widely supported yet, so it might not have all the features and extensions you want for it quite yet.
At the end of the day, define your goals for your Forum and move from there.