The only one I have been to is Comoros, twice in the last two years. Sao Tome is next on my list.
Playing around with world tourism arrival data, collected by Worldbank from all kinds of sources, is a lot of fun. And instructive. A list of least visited countries over the last 3-5 years is enough to give wanderlust to the most travel-hardened amongst us.
Which ones have you been to, and which ones do you want to go to?