6809
My favorite processor of all time. Wrote tons of code for that CPU.
After I got a real assembler, I got a copy of Hendrick's Small-C Compiler and wrote a replacement back-end code generator for the 6809 instruction set, bootstrapped it so that the compiler ran native on the 6809, grafted a floating point package onto it, and used it to write the application software for a DSP-based spacecraft attitude control system. If Motorola had manufactured the 6809 with radiation-hard silicon, that code would have flown; as it was it was relegated to ground-based prototypes and test systems. But it worked great.
Fun times, before the IBM-PC and the 8086 ate everybody's lunch. :-)