I prefer creative and artistic levels, as well as the simple, symetrical, and organized levels. Here are some of my favorites:
I liked the spinning circles for some reason:

I liked the swinging tail in this one:

Artistic...kinda cartoony actually :)

Very symetrical:

Simplicity:

Wow, i hadn't seen this one until you mentioned it, very cool:

I'm amazed what people have come up with, I never even considered making a level non-robotic-boss-ish when I first started.
(ok, maybe I went a little over the top with all the screenshots...o well...)
Feel free to post your own favorites.
~MD