France
Widely considered the toughest trek in Europe the GR20 follows the spine of the Corsican mountains from the north west to south east of the island. The trek, mountains and views are spectacular. The hut infrastructure is good and the people are very friendly.
Some people go out and do as much as they can of the route in a week. It can be completed in less than 2 weeks but give yourself 1 week more and you can take the 'alpine variations' and a few side trips. Treat yourself to some time in Corsica at the end of trek to enjoy beaches, good food and powerful cheese.
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