My recommended CPU for learning assembler is the 6502 (no surprise there), given its small, simple instruction set and very little in the way of "distraction".

My recommended CPU for learning assembler is the 6502 (no surprise there), given its small, simple instruction set and very little in the way of "distraction". I do admit the Apple II series is probably a better learning platform than the VIC-20 or Commodore 64, given the built-in monitor and the published ROM source code.