TBH, my favorite micro is the 32-bit M68000. The register arrangement pleases me, and it's incredibly easy to program for with every instruction you could need in a basic CPU. That being said I have never really programmed much with it, but that's more out of being lazy than anything else.
The Z80 is a neat CPU, but it's IPC ratio is pretty poor compared to other micros of the time (Like the 6502)
Probably my least favourite has got to be the PPC. I first off do not like reduced instruction set CPUs, but to make matters worse, the execution by AIM was terrible. They did, granted, use IBM's POWER CPU which in it's own right is not a bad chip. Even still I don't hate the PPC and it found a lot of use in Macs for years, and even the 6th, 7th, and 8th generations of video game consoles all saw at least one PPC console. Even today, the PPC line is still used by the Amiga.