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)
you need to look at the 'center' of the country though, so although France is certainly "closer" than Switzerland, since Switzerland is tinier its center is less far away