While I can't offer a suggestion for the best case, my advice when buying a clear case is always go for TPU. I have never seen a clear polycarbonate case that does not get really scratched very quickly.
The best one I have found for the black iPhone is the Poetic Atmosphere. I got a no name version from ebay for 7.99 and its my favorite case by far. Now for the white iPhone a lot of people like the frosted clear diztronic.