ME and 2k are the cutest, but 3.1 is probably my favorite after those two

admittedly, I've probably ended up spending more time with Windows 3.1 than ME or 2k (both replaced very quickly on my own systems by XP, while 3.1 has ended up being used more under emulation and I've probably had a 3.1-tan background on almost every install)
never been super big on XP-tan for some reason
95-tan is pretty good too, although I think I've seen less 95-tan pics than 3.1-tan ones

this is the real power of lolibaba ヽ(´ー｀)ノ