The very first one would have been the Atari 8-bit system running on my dad's Atari 400.
I was there, Gandalf....
I was there, Gandalf....
DOS XL, which came with my Indus GT drive I used with my Atari 800. It was menu-driven, but did have a command line interface.
The first PC OS I used was OS/2 Warp 4 when we got computers (not just mainframe terminals) at work. My first post-Atari home PC had Windows 98.