I voted XP, because it's the one I'm currently using, but my preference goes to Windows2000 pro for stability/right management (compared to other windows).
I tried windows 7/vista for 1 hour, I couldn't find where or what I needed to change to create shared folder permission over lan. I gave up and used an USB stick.
Too many "I make you feel like a noobish child by not prompting you any option .. you don't need them at all. You might not be good at informatics to use that anyway, so it's not included, don't look for it".
I really don't like the way microsoft create their interfaces, more and more for noobs. no more options available quickly, that's horrible.