IMHO the ATmega chips (on the Arduino boards especially) are the easiest and most fun way to learn how CPUs work as they're 8-bit RISC so they have have a really basic and simple to understand architecture similar to that of Apple II's MOS 6502 or Intel's 8008, but have modern documentation and toolchains.

I can easily recommend an Arduino or a clone instead of a Raspberry PI to anyone wanting to get their hands dirty.