For each country, give the name of the country that is closest to it. The distance between them is the one between their geographical centers
Data from Portland State University (https://www.pdx.edu/econ/country-geography-data), for missing countries from CIA (https://www.cia.gov/library/publications/the-world-factbook/fields/2011.html)
Interesting how by using the "geographical centres" method you can end up with a "closest country" that doesn't border it! For example, Lesotho is considered closer to Swaziland than either of its neighbours South Africa or Mozambique; Tunisia and Libya are both "closer" to Malta than each other, and Morocco isn't "closest" to its neighbours either.