Here's an overview of the countries in the top 170, each time with the top 3. Italians have the advantage of a GT and many (semi-)classics in their own country.
41x Italy: Coppi, Bartali, Moser
32x Belgium: Merckx, De Vlaeminck, Van Looy
23x France: Hinault, Anquetil, Jalabert
18x Spain: Valverde, Induráin, Contador
9x Netherlands: Zoetemelk, Raas, Janssen
7x Switzerland: Cancellara, Rominger, Kübler
6x Germany: Zabel, Ullrich, Altig
4x UK: Froome, Cavendish, Wiggins
4x Luxembourg: Frantz, Gaul, Faber
3x Ireland: Kelly, Roche, Martin
3x Colombia: Quintana, Herrera, Bernal
3x Australia: Evans, Anderson, McEwen
3x Norway: Kristoff, Hushovd, Boasson Hagen
2x Slovenia: Roglič, Pogačar
2x Denmark: Sørensen, Fuglsang
2x Russia: Menchov, Tonkov
2x Portugal: Agostinho, Costa
1x Slovakia: Sagan
1x USA: LeMond
1x Kazachstan: Vinokourov
1x Poland: Kwiatkowski
1x Moldova: Tchmil
1x Ecuador: Carapaz