I voted other. I would use the OS's I am currently using now. Slackware Linux, Apple OS X, Serinity eCommStation and yes Windows XP Pro, I'd probaly try Windows 7. Edit: I forgot, since they would be free and run on my existing hardware, HP-UX without a doubt.
Windows. Linux is not too bad, I like it, but it needs again to learn to much to use really it, I'm not interested to Unix.