I've used:

TRS-DOS (Color Extended Basic version on a TRS-80 Color Computer, model I)

Unix System III, System V

MS-DOS 3.1, 4.0, 5.22

MS-Windows 3.0, 3.1, 95, 98, 98se, Me and XP

MacOS 8, 9, 10

FreeBSD

Linux (Debian, Gentoo, lots and lots of other distros)

My favorite: Ubuntu Linux. It's got a good balance of stability and current features and it's the easiest OS to install and set up that I've experienced (at least on the 8 or 9 systems I've installed it on)...and it's FREE, in both senses of the word.