Which one?
Seriously, there are a number of them.
I use KDE 3.3.x. I find little to complain about.
Granted, it's not the best GUI ever, but neither is Windows, or Mac OS.
The best design for a GUI, IMO, was the OS/2 Workplace Shell. It was not always implemented in the best way, but it (and the underlying System Object Model) was, again IMO, the best foundation for an object-oriented GUI ever.
I wish that IBM could open up the source for SOM/WPS and give it to the KDE developers.
Mac OS X is arguably the best commercially available GUI so far.
If you have a different GUI in mind, what is it?
Have a look at Cocoa - Apple's object-oriented framework for software development. It is an elegant solution for development with the Mac OS X environment, with the "InterfaceBuilder" application for GUI design, and XCode IDE for development with Objective C++ and Java.
Cocoa is based on the award-winning "NeXTStep" development system.