Brian W. Kernighan, 1978
The smallest complete program that proves a toolchain works. Kernighan first used the phrase in a 1972 Bell Labs tutorial for the B language, where it had to be spelled across three variables because B could only hold four characters in a constant. By the 1974 C tutorial it was one line, and in 1978 it opened the book that taught a generation C.
It is lowercase, has a comma, and has no exclamation mark. Nearly every language since has introduced itself the same way, which makes this the most copied program ever written and the one nobody thinks of as an artifact. Read it as one and the whole shape of a C program is already there: the include, the entry point, the call, and the escape sequence.