I just want to add the first four climbs the Tour has ever visited (though one of them was climbed very recently).
1903:
Col de la Republique (7 km 6%)
Last climbed in 1997 and 1978
1905:
Ballon d'Alsace (9.1 km 6.6 %)
Last climbed in 2019 and 2005
Côte de Laffrey (6.8 km 9.1%)
Last climbed in 2010 and 1989
Col Bayard (2 km 8.1 %)
Last climbed in 2015 and 1991
Another classic in the past was also Col de la Faucille (11.8 km 6.2%)
Last climbed in 2004 and 1994
1903:
Col de la Republique (7 km 6%)

Last climbed in 1997 and 1978
1905:
Ballon d'Alsace (9.1 km 6.6 %)

Last climbed in 2019 and 2005
Côte de Laffrey (6.8 km 9.1%)

Last climbed in 2010 and 1989
Col Bayard (2 km 8.1 %)

Last climbed in 2015 and 1991
Another classic in the past was also Col de la Faucille (11.8 km 6.2%)

Last climbed in 2004 and 1994