Take it from a developer, Windows XP.
Any developer will tell you windows XP out classes 7 by a long shot.
Windows XP uses the least amount of ram and can run video games faster.
The only thing it doesn't support which it's starting to is Direct X 11.( which only Microsoft specific games use, marketing technique sheesh )
Windows XP also cost less memory than 7 and using small programs you can make XP 100x more godlier by customizing it to your liking even changing the desktop button with a resource tuner.
Windows XP is so far and will be for awhile the best OS. Maybe in the future Microsoft will realize Windows XP is their best card and go back to it, however I doubt it.
Another big marketing reason for 7 is due to the eye candy, but with it's eye candy requires high spec computers which end up working as well as models from 2 years ago, rather than how it should due to the OS and its ram hog.
Tada~