Hmmmm. I own around 50-52 OSs. I use all of them depending on what I'm doing. Recent OSs I'v used:

Linux...Almost every distro you can think of.

Windows....1.0 - Server 2003.

MS-DOS 5.X-6.X

PC-DOS 6.22

BeOS Pro 5.0

Free BSD 2.X - 5.X

Net BSD

Solaris 9

I do alot of formatting and OS installation. I like Windows XP, Windows server 2003 Enterprise Editon, BeOS, Linux, BSD, and UNIX.

Someday I'll actually use Mac OS X and let you know what I think.