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.

I created my little memory-wipe routine with POKE, and boy was I proud when it worked! Cool username, btw

32-bit ARM is a pretty good assembly language. It's more orthogonal than the 68000, which has separate address and data registers, and there are cheap single board computers that use it. Unfortunately, the Cortex-M series uses Thumb which is not a good match to learn assembly, so you still need a bootloader, a kernel etc.

I cut my teeth on the 68000 (even made my own motherboard) but I must say that writing assembly on the ARM architecture is pretty agreeable.

The main idiosyncrasy with ARM assembly that bothers me is how immediate values can only be values that you could obtain with an 8-bit number + 4 bit rotation. So for anything else I have to use a literal pool somewhere nearby my code.

Still, I think 32-bit ARM is the best architecture to introduce to first time assembly students.

If anyone out there wants to learn it, get a $10 Orange Pi and Pyeatt's "Modern Assembly Language Programming with the ARM Processor" textbook. Hell, you could just get Qemu with a Debian armhf image if you don't want to buy the board and search for free ARM assembly tutorials.

64 bit ARM isn't terrible in my experience and in some ways is better - for example it does away with the conditional versions of every instruction.