I voted Mac OS X. My favorite setup is a 24'' iMac with an attached 22'' monitor. I know that most people think that OS X is for n00bs and such, but I actually find that it's the easiest OS to customize and create some really tight workflows. It's keyboard shortcuts are also second to none.
Before anyone brings it up by the way, my Mac runs Crysis ;P
That being said, I do also own an old HP TC1100 Tablet PC. The little guy's got an ancient Pentium M processor on it, and only 512 ram, but I still went out of my way to install Windows Vista on it. People give Vista way too much crap.
The tablet's also dual booted with Ubuntu on there running the new Netbook Remix packages. It's nice having linux on there for when I quickly just need something done.
So... Maybe there should have been an all of the above box XD? For me, operating systems are just a means to get things done. I don't have to be on a specific system to be able to use it, and they all have their strengths and weaknesses.