I have a few cases for my iPhone and the Otterbox Commuter TL is probably the sturdiest. I wouldn't trust it completely though, ultimately nothing will protect it from a serious drop onto a hard surface, and unless you're using a case with a lid or a pouch, the screen is always vulnerable. I know several people who've dropped the phone and it's always the screen that's smashed while the rest of the phone is fine, and they use cases.
When I upgrade next year to an iPhone 5 I'm going to get one of those lovely crafted wood cases, although I've been put off Grove (despite the amazing etching) by a couple of reviews saying the parts have a tendency to become loose. Not particularly durable, but I baby my phone and I only seem to be clumsy with it at home where there's carpet and I needn't worry too much and most cases take away from the looks. The wooden cases look siiiiiiiick though.
I also use a BodyGuardz invisible skin which IMO is superior to the Zagg one and keeps the phone scratch-free whilst being... well, invisible.