Anything linux.
Debian unstable specificly, although Fedora is a close second (displacing Gentoo). But that can change on a whim.
Probably pick any of the *bsd's, I just haven't because Linux does everything I want and does it fairly well.
I like the general
unix philosophy a lot in software design.
I beleive that making a OS nothing but a bunch of gigantic do-all programs with lots of features and dialogs may make it initially easier to use, but drasicly cut down on the overal usefullness. By dividing tasks and roles into a bunch or interrelated tools that the user can directly manipulate will make it harder to use at first but will make a much more powerful and flexable system. Once a user understands how to fit the pieces together that user can do things faster and acheive higher overall quality then otherwise would be possible.
Then again you don't want to get bogged down with trivialities (allowing laziness thru automation are wonderfull things in a OS), a quick GUI is great as a front end to those tools and makes work easier, just as long as it doesn't interfere with the ability of a user to manipulate most parts of the OS at the various levels of abstraction. (which is a very hard thing to design a GUI to do)
OS X is good and enjoyable to use, but not Free (don't care about $, it's as in freedom) and the big overcontrolling GUI and related systems are annoying after a while so I am not going out of my way to use it, but I would like to definately dual boot it because I like how Apple does the UI and would like to learn from it and I admire the foward looking nature of the technology they include in their computers.
It is a nice diversion from normal *nix way of doing stuff once you get over the constrictive nature of Mac OSes (even windows users feel constrained at times.) it's nice to use. (go with the Apple flow)