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.
Lowercase, a comma, 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. Look at it as one: the include, the entry point, the call, the escape sequence. The whole shape of a C program is already here.