If you believe Popular Mechanics and their metrics, it's the USPS. They measured the number of sudden drops, among other things, and the USPS came out on top.

One disheartening result was that our package received more abuse when marked "Fragile" or "This Side Up." The carriers flipped the package more, and it registered above-average acceleration spikes during trips for which we requested careful treatment.

Via TAPPED and Matt.