I might also suggest ARM. Way back in the day, one of my first-year CS modules included writing a rudimentary MIPS emulator for much the same reasons as above, but I came to that knowing 6502, x86 and ARM2 assembler already, having cut my teeth on Acorn stuff, and at the time I still preferred ARM2 out of all these for clarity. ARM obviously remains relevant today, albeit with many revisions.
