Windows 7, used had Vista but replaced with Windows 7 due network bugs in 64 bit that I couldn't deal with it and they don't slow my computer down after installed many games.
I love Windows 7, I can say that honestly. Vista was a pain in the ass but so far I have had no problems what-so-ever with Windows 7 ultimate, it even has a simulator to run programs not compatible with Windows 7 but still compatible with another version of Windows.