Otterbox seems to make the most protective - if not the defender (big. bulky, awkward, but great protection), the commuter is a nice compromise.
Personally, I think a soft rubber case is better than a hard case. The single most frequent damage it seems from dropping an iPhone 4 or 4s is the screen shattering as the edge of the glass strikes something. If you have ever messed with corning glass, you realize you can hit it with a mallet on the flat surface with no ill effects, but bang something metalic or very hard sharply against the edge, and it shatters readily. So a rubber case that wraps around the whole outer edge, and covers the edge of the glass, seems best. A hard case seems more likely to just transmit the shock through to the glass.
The otterbox commuter is a silicon wrap around case, with a hard plastic outer layer that slips over that. It offers good drop protection without being overly bulky or awkward.
Personally, I hate all cases and do not use them. I prefer a leather slip case (Sena kutu) but that way when I take it out to use it, I have a bare iPhone.