The Z80 I enjoy because of nostalgia, largely. I started with a Z80-based system:

https://blog.steve.fi/how_i_started_programming.html

The instruction set is simple, and clean. The fact that you don't need much supporting circuitry to drive it is a nice bonus, but mostly I remember coding the thing when I was 12 or so, and now I know what I'm doing for real :)