His record doesn't bear that out.
He hasn't done badly though:
- Windows XP
- XBox 360
- Windows 7
- Office 2007
- Office 2010
- Halo (via Bungie)
- .NET and SQL Server
- Internet Explorer (post IE8)
- Windows Mobile (although it outlived it's appeal)
- Vista (personally didn't mind it, but it came short of what people were expecting from a 6 year wait)
- Windows Phone (not the OS itself, but the fact that they waited so long and now have to play catch up, major screw up)
- Expression developer tools
- Internet Explorer 6, 7, 8
- Windows Live Social Networking BS
I didn't include Windows 8 because we don't really know how that's going to turn out yet. It may be a gamble that pays off, but IMO Ballmer has done good by the established Microsoft cornerstones (OS, Office, Developers) and made themselves pretty much the biggest name in console gaming. Where he's repeatedly failing though is steering MS into new markets, or markets that MS have left open for competitors (i.e. smartphones, web browser). Overall though, profits are up year-on-year, MS are barely losing ground in their established markets. They couldn't ask for much more in a CEO.
Is he the best CEO ever? Hardly, but he's far from the worst. I mean, he could be Trevor Moore (UK business man who, in CEO positions, helped steer two of the UK's biggest companies into administration).