The one in the Korok forest where you have to figure out the relationship between the star map and the stones took me a long time and felt so satisfying to complete.

The game is polarising because it works only for a certain kind of player. I love just jumping in a random point of the map to chill and explore and suddenly find some surprise in the middle of nowhere, but if you prefer some linear and story-driven game I don't think BOTW is for you.