I normally use Windows, just because it's the most common OS and I help a lot of people w/ their computers, so it's good to have a lot of personal experience with the same OS as them. I'm currently running XP Pro on this computer, and 2k pro on my other one (networked of course). I have a couple distribs of Linux, but neither installed ATM. I will prolley get a distrib of BSD just to try it out later.

I have no personal favorite, because in my opinion it depends on what you're doing as to what OS is best.