I like levels that are fun to play over and over. Usually that means they have some ability moves. A little randomness helps too, so gameplay is never exactly the same twice (and often pretty funny). Here's a few that I play a lot that are different and random and fun: Perfect Storm by @JackBauer. Beautiful, wonderful design and wild action: Rough Crowd by @Check Mark. Chaotic and exciting: Mad Bull by @Akira Wong. Creative and odd and ingeniously simple:
Here's one I remade into a random level. The original, Nautilus, was for kids by @stefanovime. To make it more challenging I ended up using more of the Rs to make it random (and fun, I think).