I voted for DOS, it was the first OS I learned, and when I say learned I mean it... I knew what every file did, what could be deleted to make room on the hard drive. I was very fond of DOS 5 it was the first one I really got my teeth in to. I liked DOS-Shell, strange I know but it was useful, especially for my wife. DOS 6 was a bit on the big side, see we complained about bloat even then. It did bring drive compression, a little unreliable but a God send for the small dives we had back then. 6 also gave us boot menus in autoexe.bat and config.sys. My 286 would boot to a choose of several configurations. Max memory, straight in Wolf3D, another for doom. One for the wife (DOS-Shell) and one to my office package, the name of which I can remember. Later once I has a 3.5 inch 1.44Mb drive to go along side the 5.25 one I could install Windows 3.1 I still used the boot menu for the games to cut Windows out of the loop, saves RAM, 2Mb doesn't go far...
I still revert to the command line sometimes even now. Nothing like IPCONFIG /ALL to really find out WTF your network is up to. Also when I'm repairing someones PC I like to use the command line a bit when they are watching me work. It always impresses the hell out of them, and they think that they are getting their moneys worth

"Cos it this geek bloke has to "hack" the computer to make it work then he must be good, and there was no way I could have repaired it myself with system restore"