Probably among the least visited countries are small island nations that are difficult (and expensive) to get to. And especially because of coronavirus, these countries are closed in order to protect their small populations. It is easier to close an island country of course. I’m thinking of Nauru, Palau, Micronesia, Marshall islands, Tuvalu, Kiribati, Tonga and Samoa in the Pacific. (Without coronavirus, Fiji and Vanuatu get many tourists.)
Sao Tome and Principe off Africa in the Atlantic and Comoros off Africa in the Indian Ocean are island countries that are hard to get to and are not visited much.
For political reasons and reasons of war and violence, I would say Iraq, Syria, and Yemen. Also Palestine, Central African Republic, South Sudan, Afghanistan, Somalia, El Salvador. This of course is a shame. Other countries get a bad reputation (not deserved) for one thing or another; I’m sure you know which ones.
Others have small populations and haven’t advertised themselves: Guyana, Suriname, Moldova, Belarus, certain countries in Africa (Mauritania, Chad, Eritrea, just to name a few) don’t get many visitors.