I vote for microcontrollers. I learned assembly on Atmel's AVR and it's was easy and straightforward because there's very little abstraction underneath, there's no OS, no heap, no multiprocessing and no context switching, no syscalls and no fat libraries, and you get direct access to the hardware. You also receive actual physical feedback — doing a tiny bit of bit fiddling gets you a blinking LED or whatever.

AVR's assembly is quite mediocre, with 120+ something instructions, with lots of duplication among them (IIRC — it's been... many years already), and some people swore by PIC which only had 35 instructions to remember. But it was still easier than lobotomizing oneself by trying to write a Win32 application in x86 assembly (which came later... and went to the trash bin quickly while microcontrollers stuck for much longer).