Overall, OS X 10.4 Tiger reigns supreme in my book; just the right amount of features without useless eye candy, (but still tasteful) and solid as a rock.
On the Windows side, I'm partial to Windows 95 because it was what my first computer I owned ran. But, taking off the nostalgia glasses for a moment, I must admit that BSOD and I were good friends.
Windows XP was truly the best overall; simple design, and decent stability.
Linux? Uh. Linux Mint was...nice. Ubuntu is, you know, um, popular and sort of...pretty. I think I used Damn Small Linux once too?