Vaguely related, my recommended CPU to learn assembler with is still the 68000, whereas my recommended platform is still the Amiga.
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.