It's fun how most people just hate the hardest levels.
In the original game, I can't think of a least favourite level or world at the moment. In Rayman 2, the Top of the World was pretty much a filler level so I'd choose this one, even though I'm always tired of playing the Fairy Glade every time I restart the game. In Rayman 3, that would definitely be the Longest Shortcut, which I never found inspiring, in addition to being quite boring in my opinion. Not sure about Origins, but some of the icy Gourmand Land and Grumbling Grottos levels just don't make me feel like playing.