What’s the most executed piece of code of all time?

"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.

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?