1. As of TAR27 Leg 4, all countries starting with the letter Z (Zambia and Zimbabwe) have been visited in the US version! This is the 2nd letter where all countries have been visited, after the letter O, which only had 1 country, Oman.
Just 1 more for the following letters...
F - Fiji (Finland and France visited)
J - Jordan (Jamaica and Japan visited)
Q - Qatar (There are no other countries starting with Q. Q is one of 2 letters which 0% of countries have been visited.)
R - Rwanda (Russia and Romania visited)
Y - Yemen (There are no other countries starting with Y. Yemen is just too dangerous to be visited.)
2. If you arrange all UN member states in alphabetical order, the first 6 countries have yet to be visited in the US version. They are Afghanistan (0% chance), Albania, Algeria (nearly 0% chance), Andorra, Angola and Antigua and Barbuda. Of all countries visited, Argentina is the first.