I agree that M68k is nice, as are the 8-bit ones you mention. I just find it strange that you like Z80 and dislike x86 - they are fundamentally not that different and both are descended from 8080.
Yeah the Z80 instruction set is quite messy (mainly because it had to fill gaps of the 8080 instruction set for backward compatibility). But as an evolution of the 8080 instruction set, the Z80 is still cleaner than x86 (IMHO!).