i would have to say rubber cases.
i recently bought a plastic case for my phone, I have the Samsung Upstage, and after one drop the plastic began to brake. after the second drop it would not clip to my phone at all.
My boyfriend has a rubber case for his blackberry curve and it clips to his belt buckle. He was riding his motorcycle and going fat, like 90, and it fell off of him. He noticed it like an hour later and went looking for it. He found it in the middle of the road and the case had some scratches on it but the phone had like 3 scratches and no damage done to it.