Mac OS X is my favorite. I hate Windows 7, it is way over complicated.

I think that the fastest and most efficient overall OS is OS X... I have Windows 7, Linux, and Hackintosh running on my PC at my house, and Windows 7 is terribly glitchy, Linux I don't mind, but OS X is amazing.

Windows 7 Ultimate 64-bit

Right now I'm on a Kubuntu 64 system, but I've used plain Ubuntu, Arch, Sabayon, and openSUSE before on here. Probably going to try Chakra or Pardus next. (I would use a BSD if it worked with my hardware) I don't just mean in a VM either, I mean full on installation.
Picking a favorite would be kind of silly and pointless as all have their strengths and weaknesses, though I favor Linux in general over Windows or Mac, and I use all three most every day.

linux is the best.
what distro depends what you want.

Server - Red Hat
Fast and Light Weight - Ubuntu
KDE - Mandriva
Eyecandy - Debian/Ubuntu

LOLWUT!?!

I don't know much about servers so I won't argue Red Hat, I hear it's good

Fast and Light Weight is not Ubuntu, nothing with a desktop environment, save for e17, is light. You gotta use a wm like awesome or fluxbox to get lightweight.

I never liked Mandrivs's KDE, too KDE 3 ish. But that's just personal preference, and easily changeable.

If you think Debian has eyecandy, you have never installed Debian. Ubuntu is different, especially with Unity which is quite pretty. (I hate Unity, but that's another subject)

I like Open Suse 11.3

@DucTape
if you think you cant get really good eyecandy for debian then you dont know how to use debian.
ubuntu is light compared to alot of gnome systems with all the useful shit ubuntu has.

Look face it. 100% of Operating Systems are crap...

I say screw operating systems, what have they ever done for us!

@massey101
Correct! Command prompts are the way to go :D