Comment by TangoTrotFox

Hahaha, mine:

    mov ah, 2
    mov dl, 7
    int 21
    int 20

Discovering that I could write my own programs with debug.com was earth shaking. Picked up some decade old book on it at Half Price Books (which went well with my decade old machine -- an 80s model IBM with integrated monochrome green screen) and never looked back.