Slate has a great article on the most impactful code in programming’s short history:

Code shapes our lives. As the venture capitalist Marc Andreessen has written, “software is eating the world,” though at this point it’s probably more accurate to say software is digesting it.

Culturally, code exists in a nether zone. We can feel its gnostic effects on our everyday reality, but we rarely see it, and it’s quite inscrutable to non-initiates. (The folks in Silicon Valley like it that way; it helps them self-mythologize as wizards.) We construct top-10 lists for movies, games, TV—pieces of work that shape our souls. But we don’t sit around compiling lists of the world’s most consequential bits of code, even though they arguably inform the zeitgeist just as much.

To me the following line of code - published I believe for the first time in Kernighan & Ritchie’s “The C Programming Language” in 1978:

1
2
3
4
5
6

#include <stdio.h>

main()
{
  printf("hello, world\n");
}

As you can probably guess - this simply outputs hello, world onto the computer screen. This simple 6 line program, or the equivalent of it in your chosen language, was the first step in every programmer’s journey.

How Slate did not give it the number 1 spot is beyond me.