My hand. But seriously, an otterbox defender, speck candyshell, belkin sheer candy (don't know if that's the right name).
Thinking about getting a seidio obex. Waterproof, slim and safe. I have a kid and another one on the way.
Spigen NEO Hybrid EX Slim is one of the best iPhone 5 bumper-style cases around. It includes a set of three jelly bean home buttons, excellent full coverage front and back protection films.
What colour do you have? I was extremely tempted by the red and yellow variations, but in the end I was worried about it looking too gaudy, so I went for one of the metal-effect ones.
The black bumper on the white iPhone 5 looks solid, makes me feel better about being a guy with a white iPhone. The screen films are the best I have used and 3 for a buck means you can mess up one if necessary.
Speck Candyshell. I bought a UAG Urban Armor but it didnt fit my phone well and had a large gap at the camera hole where dirt and junk could get in so I returned it.