The general consensus of opinion usually names the GR20 high-level route across Corsica as the toughest of the Grande Randonees (SP), but with some justification the Pyranean Haute Route – which teeters across the frontiers of France and Spain – gathers a lot of votes.
GR20 Corsica - by Catching Flies
Pyrenees - click for more photos from the Wandering Ostrich
Both are long and demanding, have sections without proper paths, and journey through remote country, often far from habitation.
Yet there’s another long and challenging route that is also a contender for the title …
Who would know?
Cicerone guidebook author Kev Reynolds:
GR20 Corsica - by Catching Flies
Pyrenees - click for more photos from the Wandering Ostrich
tourism-oisans website
All 3 sound great to me.
These are routes that have guidebooks and maps. Not off trail adventures.