My favorite is a mobius strip grown big and turned into a shawl/cowl (aka scowl). I've finished one and almost completed the second. The whole thing is supposed to represent something I love and is full of things to represent numbers (Fibonacci numbers, prime numbers, golden mean).
Here are a couple of pictures (the model is my gorgeous niece).
I have a second one almost completed in a beautiful silk/merion blend from Malabrigo. It's my sister's gift and she chose the yarn.