I guess it's XP for me:
Having discovered the joys of nLite & M$'s less well publicised XP-variants I've got to say I've found a mainstream OS I can live with...
...best emulator support.
...surprisingly low overheads (after some work).
There's OS's I MUCH prefer but there's no software.
Oh, dear I wonder if I'm an XP fan-boy..?