Arguably PDP11 is probably the best. Dates back to the assembly language programming era, but nice clean instruction set. But what are you going to run your PDP11 code on? A simulator? Sure. But no real, consumer facing machines with graphics chips.
If you like the PDP11 instruction set, then you'll probably enjoy 68000 as well. It drew a lot of inspiration from the former. You lose the "deferred" addressing modes (though the 68020 adds something similar), but gain access to a few other useful ones. Conveniently it was used in a lot of consumer devices (Mac, Amiga, Sega Genesis, etc.)
