I'd recommend the BBC Micro, since we're not talking about the 1980s and geographical limitations of the market, the BBC has a nice basic that includes a built-in assembler, has a text UI that is more pleasant on the eyes than most 8bits, has the sideways rom system to make it possible to add a debugger and other niceties, and has a simple easy-to-understand disk filing system.

6502 itself is rather simple, I wasn't so much interested in game programming as hardware programming, working with the TIA and PIA are the interesting bits to me.