A chimpanzee. Imagine a very intelligent pet which can even learn to communicate with you. And you could easily build a cool cage for it with enough space. I always feel bad when I see them in the zoo...
I guess the micro ape would be a bit scary, but it's still worth it ;).
You win a lot of money but are forced to donate a part of it (let's say 10000 $) for good cause. Who should get it?