I know I'm going to get eaten for this, but for me the worst so far has been the original apple bumper. The thing fits loosely, looses its shape, and offers virtually no protection whatsoever. In addition, I absolutely DETEST the integrated buttons on the case. Yes, I know people have been able to "boil" them back into shape...but 30 bucks for a case I need to throw in a pot with my macaroni to make it work? No thanks.
So far my best experience has been with the body glove vibe slide case: nice looks, solid feel etc. I have an alumacase and an atomic billet both on the way, so we'll see if they live up to their hype and price.