It's really not a question that interests me, or makes any sense as far as I am concerned.
Which OS would be my "favourite" would very much depend upon the computer that I am putting it on and the use that I want to make of it.
Pretty much what ijack said. It depends on the hardware and my needs. I love Windows 7 and think it's the best OS Microsoft has released. I game a lot and am overall a fan of the Windows environment.
OSX is an excellent OS, too, and runs very well on a Mac. Clients/friends that have bought Macs find it easy to use and learn. I prefer it for graphics work.