I've learned Z80 and 8051 a decade or two ago, and then forgot everything. Honestly both were easy to pick up, but I assume you're opinionated and/or an expert?
Anything in particular that you like about the 6502?
I think people like the way the 6502 wires up to peripherals. Myself I think the Z80 is much better because it has enough registers and addressing modes that you can write compilers for it. I know they had C compilers for it in 1984 because I typed in a C program for CP/M from Byte magazine and got it to run on my 6809-based TRS-80 Color Computer. Programming languages for the 6502 were usually implemented with virtual machine techniques like
because it is way faster, has a bigger address space, and has wider registers so you can do pointer math over that bigger address space unlike this turkey
Anything in particular that you like about the 6502?
https://en.wikipedia.org/wiki/SWEET16
or the truly atrocious
https://en.wikipedia.org/wiki/UCSD_Pascal
which was one reason a generation of programmers hated PASCAL with a passion and declared you could pry BASIC from our cold dead hands.
Myself I'd want to hollow it out and put something based on
https://en.wikipedia.org/wiki/Zilog_eZ80
because it is way faster, has a bigger address space, and has wider registers so you can do pointer math over that bigger address space unlike this turkey
https://en.wikipedia.org/wiki/WDC_65C816
A lot of people enjoyed writing assembly for the 6502 back in the day though.