6502 will net you the Atari, NES, C64, Apple II and others. Its also a nice lead-in to the 65816 found in the SNES and Apple IIgs.
z80 will net you the Gameboy and Sega Master System, among others.
68000 would net you the Sega Genesis, and some of the early home PCs like the Atari ST, Amiga, and early Macs.
Then you have ARM of one version or another in all the game boys since the Advance.
The SNES I think is actually not really well documented as far as homebrew compared to others like the Genesis, NES, Commodore, Master System, or Gameboy -- mostly because the hardware isn't entirely off-the shelf -- but of course its the most capable of any older platform that's approachable, and has a special place in many people's hearts.
It's probably worth mentioning that the Gameboy uses a heavily modified Z80 that has had several instructions and two registers (along with their associated addressing modes) removed, and a few instructions added. (Similarly, the NES and SNES CPU's have removed the 6502-family decimal mode.)
The SNES is actually very well documented - the big difficulty with SNES development is that the hardware is rather more complex than other 2D consoles, what with having 8 video modes, semi-transparency, 2 arbitrary clipping windows, and so on. Additionally, using audio requires writing code for an entirely separate CPU, the SPC-700, that must be transferred to the sound chip via a serial bootloader because the audio RAM is not mapped into the main CPU address space. (Genesis audio is somewhat easier in that respect, in that while Genesis audio is nominally driven by a separate CPU, that CPU's RAM and the audio chips themselves are mapped into the main CPU's address space, and the cartridge ROM can be mapped into the audio CPU's address space.)
If the OP would prefer working on the SNES, the NES would be a good place to start, as almost everything you learn from it will be directly applicable to the SNES. The SNES hardware is, for the most part, NES hardware heavily scaled up - a CPU with expanded registers, address space, and instruction set running 50% to 100% faster, controllers using the same transfer protocol but with more buttons, a graphics chip with the same basic architecture but four times the memory bandwidth, and more powerful and flexible DMA hardware. The only really major difference between the two is the sound system architecture.