It depends on what you are looking for. Are you looking for maximum protection? If so, then the Otterbox Defender is the way to go, but it makes the phone very bulky. But, unless you drop you phone and it lands screen first onto a very sharp corner, your iPhone really can't get hurt in this case.
If you're looking for something that offers decent protection, but is still slim and keeps the styling and lines of the phone, you may look into a case such as Seidio's Innocase.
If you are looking for a blend of these two above, I'd suggest looking into either the Speck Candyshell cases, or the Otterbox Commuter cases. They offer very good drop protection, while still looking very stylish and keep the lines of the phone. And the Otterbox Commuter also has the added benefit of having built in headphone jack opening and dock connector opening covers, so that lint and/or moisture can't easily enter the device through those holes.