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!).

Also, the Z8000 looks quite interesting and like the better 16-bit alternative to the x86, but it never took off: https://en.wikipedia.org/wiki/Zilog_Z8000