My favourite region in the game was Lanayru, and my least favourite region in the game was also Lanayru.
You see I loved the Lanayru Sand Sea, it was extraordinary to explore those islands and the arias in-between. I've always loved the sea, so maybe that is the reason I liked that specific region so much.
Then my least favourite region is the Lanayru Desert, it was a unique and grand place to be at, but at some points it did feel just a little dull compared to all of the other regions in the game.