Personally, I use Mac and Windows for several years and Ubuntu for few days and my conclusion is : there is no perfect OS.
They all presents adavantage the others don't have, and they all have stupid functions the others don't have.
All depends on what are your attempts (do you like to fiddle with your computer, do you want to have something usable immediatly or do you want to make it as you want etc...).
And many details depend on the OS version. For example, I hated : windows 98 and Vista, but I loved Windows 2000 and XP (I have never used Seven yet). I loved Mac 10.5 and 10.6, but I absolutely hate 10.7. I appreciate Ubuntu because it's fun to enter command line (it gives the feeling I am a big nerdz XD) but I spend most of the time to feel "Shit, why doesn't it work ???", than having pleasure with Linux (even if, when I finally succeed to do something, I'm glad like if I'd climbed Kilimandjaro).
So, I think all depends on your way to work. At the moment, I think Snow Leopard is first in my heart, because it was usually really simple to use it. I think I have less headaches with this OS, than with any other OS... even if I really disagree Apple's point of view, in particular with Lion...
/!\ Never give your activation codes to someone else than a TVPaint team's member
/!\ Ne donnez jamais vos codes d'activation à quelqu'un d'autre qu'un membre de l'équipe TVPaint