My best seller in all of my stores has consistently been Mad People.
I've combined the Cheshire Cat drawing by John Tenniel for Alice's Adventures in Wonderland with Lewis Carroll's words to make an image that seems to resonate with a lot of people.
I have Mad People at 3 different print-on-demand services:
CafePress, Zazzle and Printfection, with a variety of products at each one.
Don't you know someone who would love to get Mad People as a gift?