>>149886319
According to PCS's "Profile Score" rating, the hardest stage ever was stage 17 of the 1977 TdF.

Which I don't quite understand because Stage 18 of the Tour this year has comparable length (172km vs. 184 km), shares 2/3 climbs (Col de la Madeleine and Col du Glandon), and the third climb of 2025 (Col de la Loze) is harder than third climb of 1977 (Alpe d'Huez).

Yet the profile score for 2025 stage is 414 vs. 1977 stage at 715