My favorite levels are the ones by Talps. He's got five or six at this point, but they are much harder than the original stories. "Eurydice" and "I Dreamed of the Sky" are my personal favorites.
"Dark Sky of Wish Mountain" is an easy, Nifflas-like level.
Make sure you play "Don't Eat the Mushroom" for some real fun (super easy)
"Don't stop running" is also a lot of fun, but more difficult.
"The Great Race" by B-Man sticks in my mind as well.
I liked "Gustav's Son" by Polana... it was fun and humble.
Once you are an expert, play the "Dark Sea" levels.
And definitely check out the Level Release subforum and the recent Epic Levels Competition. Cosmic Meltdown" and "The Tower of Lightning" were both worth playing.