I accidentally ran over my iPhone 4 before I got a case, and it came through without a scratch. I fumble-fingered my iPhone 4S and it landed on a corner and the glass screen cracked all the way across and had to be replaced. The moral of this scenario being the corners are the most vulnerable part of the iPhone and need the most protection.
I don't have an iPhone 5 yet, but I bought the Otterbox Defender case so I would be ready when it gets here. It is out, has an outstanding reputation, and appears to provide the most complete protection, even a clear hard cover over the screen. It is neither the thinnest and lightest nor the bulkiest and heaviest - but its multilayer protection is impressive.
If you are looking for a minimalist approach Zagg has a tough plastic Shield that covers the screen and back and is sufficiently small that it would allow a third party case that covers the rim to be used as well. The Zagg Shield is the only cover I have seen that can be used with a dock. On my iPhone 4S I use a Mickey Mouse imprinted case I got at Walt Disney World and it has served me well so far, but a corner broke off of my wife's.
NOTE: Many, perhaps most, of the cases I have looked at are relatively easy to put on but a bear to take off. Certainly you won't be taking the case off often, if at all. But ease of installation and removal is something for you to think about when making your choice.
