Best of Show: PDP-7 and PDP-11/40 emulation
Here is the C program: https://www.ioccc.org/2018/mills/prog.c
It is a PDP-7 emulator that can boot the most ancient known snapshot of Unix. This is running Unix from 1969 where the compiler is 'bc' (no 'cc' yet)
then, he wrote a PDP-11 emulator in PDP-7 assembly and uses the PDP-7 to emulate a PDP-11 to boot BSD 2.9.