"Piece Of code" is too imprecise. A single instruction on a microprocessor? A subroutine? As another comment implied, does DNA count as code? Or transistor logic within a microprocessor?

A related but more subjective question I used to ask developer friends was "what's the most IMPORTANT piece of code written that can fit on one screen?".

The best answer I got was rc4, written by an anonymous author but what made security, privacy, and digital commerce possible on the web.

Is it part of SSL?

Yes, and most other security standards. See: https://en.wikipedia.org/wiki/RC4#History

As in, distinguishable piece of code running on a CPU? Probably some routines in BLAS / LAPACK.

What does in code mean? So operations in a ASIC don't count, but DNA does?

I would say memcpy or similar (strcpy)

Operating system idle loop?