If you want something friendly and CISC-y and modern, check out the Renesas RX600 and friends. They have a nice instruction set and zero-wait-state RAM and ROM, so writing assembly by hand with predictable timing ought to be easy.
(author here) ...FYI I actually learned assembly language for the first time back in the 1970s and 80s using a 6809 inside a Radio Shack "Color Computer." I was super-fun at the time. I don't remember much of it now but I'm sure x86 isn't as clean or fun as 6809 assembly was.
MIPS and ARM are fun, though, AFAICT. As is 6502.