In the last month I have had to install two forums, SMF and bbPress, and deal with a forum install using phpBB. By far the easiest to install and configure is bbPress (from the same people as WordPress). The others have pages of configuration options related to all tehir features. Frankly, unless your forum is targeting geeks or a bunch of people familiar with the overblown features of most forum software then go for bbPress. It also integrates easily with a WordPress install. I had the forum up with template and functionality customized in a couple of hours. The plugin architecture allows adding features easily. Development seems to be a bit slower and quieter than most forums but this seems ideal for a simple forum.
Hint: if you want to use this for a private forum for your business check out these plugins …
Force Login to view the forum
Restrict Registration – only allow registration from people with an email address at particular domains (i.e. yourcompany.com)