Windows 2000 was my all time favourite. It was lightweight, snappy, reliable and clean looking. MS should have stuck with that platform and simply add compatibility with newer tech such as 64-bit, newer drivers, security etc... All the flashy GUI stuff is just a mess now and they keep changing where stuff is. In 2000 everything was the most logical imo. Thankfully you can return to Windows classic but I think they removed that ability in 8.
Now I'm on Linux though, while I enjoy this platform I find it has it's quirks such as lot of things being harder than they should be or stuff is lacking in certain areas. I can't say I prefer a distro more than the other, but so far I like Mint (17). The only thing I don't like about Mint is the "explorer" is sub par such as lacking a decent search (there is one, but it just plops a bunch of icons on the screen without telling you where they are in the file system), and some apps like Kate are broken but in general I don't know if I'd want to go back to Windows at this point. I like the idea of not being at the mercy of a corporation.
Windows ME was a joke, but to be fair, the only time I saw it is when someone had an issue, so the only version of ME I ever got to deal with was ME full of spyware and other garbage. But still, it just felt so unstable. I'd always replace it with Windows 98 or 2000 depending on the system specs. Same with XP, when it first came out it was terrible but mostly because it was put on hardware that was underpowered, so I'd put 2000 instead.