I have to say Windows 7. I've used 95, 98se, ME (Dear lord, it was just a virus pretending to be an OS), 2000, and XP. I've used XP since it was released, and after Vista, I thought Microsoft would never be able to top XP. After XP's service packs, and just for how long it was in use, it was, to me, an extremely stable OS, with more software written for it then you could shake a memory stick at. Then along came Win7. It was Vista, without all the crap, and was as fast as XP. I downloaded the RC, and two days later uninstalled XP and never looked back.