OtterBox has built itself into one of the most successful iPhone accessory companies by making some of the best tough iPhone cases. The Defender Series for iPhone 4/4S is a slightly modified version of the original Defender Case for iPhone 4 that now fits both models. A screen protector is integrated into the case, and you get a holster and belt clip that you can choose to use--or not. Facing your iPhone into the holster offers maximum screen protection. It's available in multiple colors. (Note: This also comes in a special Camo version for $10 more).
For those who don't need quite the protection of the Defender Series, OtterBox's Commuter and Impact cases for the iPhone 4/4S are also good options.