The most impactful single unit of software I can think of would be the C compiler.
It’s hard to find anything that runs on a computer today that wasn’t compiled by a C compiler, compiled by something bootstrapped by a C compiler or being executed on some platform compiled by a C compiler.
It’s hard to find anything that runs on a computer today that wasn’t compiled by a C compiler, compiled by something bootstrapped by a C compiler or being executed on some platform compiled by a C compiler.