DOS
If we didn't have that ugly dos command prompt, no one would have been motivated to make an OS that was graphically driven. How many years did we have (mainstream) command line based computers? 20 years maybe? I got sick of those in my early years, was glad to move to something that had pretty graphics and that box thing they called a mouse
Easy to use? Ha
Secure? Well if you don't know the commands, I guess so
Compatible? Well it runs on just about anything, only consisted of about 1 file to translate commands and a few hidden files to work with the IBM compatible machines. It would fit on a 160K floppy disk
Is it the greatest? Well only for a few decades, then it got old really quick
