Col de Vence
The Col de Vence has been the battle ground for many an epic battle in Paris-Nice. 2018 saw stage 6 go over the mountain and cause a decisive split in the race.
The climb is not in itself hard, being 6-7% gradient over about 10km. However it is very exposed, so you are at the mercy of the weather. In the summer it can feel like an oven, and in the rare cases the weather is bad, the wind and rain chills you to the bone.