I have tried them all, and I am currently playing with XP.
I have had some scarry crashes (just reboots without warning). I used ME for a couple of years without problems until about 2 months ago when it refused to run my web development software (kind of an issue for me).

Anyway, upgrading my ME installation to XP was a nightmare but starting with a fresh install seems OK (except for these silly occasional crashes)

Just thought I'd ask what other people are doing with all the OS's available now.