My first was DOS. I was the first kid on my block to own a computer (and then the first kid to have internet access) back when everything was green screens and floppy disks were 8". Space Invaders was the first ever video game (and DOS program) I ever played/used.
I remember being amazed when Windows 3.1 came out and everything was graphical and clickable. I'm amazed, today, even more at how far and quickly technology has progressed (and a little horrified).
Despite being impressed by nice UIs and detailed graphics, though, I have come to prefer the command line. These days, the only operating systems I use are Unix-inspired (Linux, mostly, but also BSD on occasion).