OtterBox claims you won't find a tougher case than the OtterBox Defender Series for iPhone 4. But a company called Ballistic is saying its $49.99 Hard Core (HC) case is "better" than OtterBox's, offering four layers of protection instead of OtterBox's two. However, it is rather bulky.
Like the OtterBox, it has an integrated screen cover along with a holster/belt clip that provides increased screen protection (you face the screen into the holster). It's available in multiple color options.
For those of you who find the Ballistic HC just a wee bit bulky, the company also has a slimmer case called the Ballistic Lifestyle ($29.99) that's equipped with interchangeable corner bumpers in various thickness options (you can go curved or rounded). It comes in multiple colors with a total of 10 bumpers (five red, five black).