I would say, Windows 7 for me. I never really liked XP, not sure why...
I find myself throwing windows about trying to get them to maximise on every machine I use...it's become second nature almost.
There are some things that I do like about other OSes though, having used linux a little, and a mac even less.
In linux, I like the way the scrolling works. (whatever window you hover over can be scrolled, active or not)
In mac, I like the way that some of the F buttons have clever features (like show all windows etc), and putting the mouse in one corner of the screen can invoke an action...
Just little things that are well thought out, that make an impression when you use them, and there are probably apps that can emulate the same, but never really looked...