The user interface for desktop Linux is dreadful. The Windows GUI is slightly better, but not enough to justify the additional expense in many cases - expecially with all of Microsoft's security and reliability problems.
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.
mash here HAL...www.linspire.com