Eh... It depends on the hardware.
Computers:
On a 25Mhz i486, I would choose (and I have) DOS with Win3.1
DOS is so cool
On a
potato
cooker (PC) I would use Slackware 13.37, but I'm stuck to using Win8 (not 8.1, because that's even worse), because It's too painful to migrate all the stuff to my Slacky installation. Also, the integrated graphics in this PC is poorly supported by the X server, so that's another reason. But at least
I made it WinExp-styled (WinExp stands for Windows Experience, aka. Windows XP).
Also, I never had an Apple product, because it's just unaffordable, so I can't really judge it.
Phones:
Cyanogenmod 7
Why that old? Because it's the highest that my device(s) support
Most of the times it has better and more features than the stock one the device came with.
Other:
I mean, I don't really like Windows for PDAs, but the bubble popping game in them is just indescribably good
Conclusion:
My favorite operating system is Slackware 13.37 and DOS
But WinExp is not bad either. I'm using it right now xD