Best: XP, 7, 10
Worst: Vista, ME
ME says it right in the title: Mistake Edition. Super unstable and broken OS.
I love XP just for the nostalgia value alone (first OS i ever used). Everything about the OS felt just right to me. Who doesn't love the beautiful luna theme and the great bliss?
7 was a great leap ahead of the crappy vista and was my OS for quite some time (a few years).
Vista was the most sluggish piece of crap ever made.
Windows 10 is a great OS which I'm currently using, it works nicely and has good features.