What's your preference in terms of writing and entering notes? Do you like using trackers or is a piano roll more your thing?
If trackers are ok than I would recommend LSDj just because is the most common, it's portable, it's easy to use, midi syncable, etc.
If you prefer more of a piano roll for entering notes like a modern DAW, than I would recommend something Like MSSIAH for the Commodore 64, Midines for the NES, and mGB for the gameboy (using arduinoboy of course)
There is also another option in terms of writing. There is the Synth based softwares. You can again use Midines and mGB(arduinoboy) as live synths using a keyboard to perform the notes live, but you can also get cynthcart for c64 and play it like a synth too. MSSIAH also has a monosynth program. One synth I've really been liking lately is Spritesmod's Gameboy Advance Midi synth cable. That one you'd have to build the circuit and flash the chip yourself but it's fun once the cable is built.
When it comes to chipmusic, there are just so so many options.
My advice would be to either start with the system you have the most nostalgia for, or start with the system that has the specs/number of channels/sounds you desire. I started with Commodore 64 because the chip was more sophisticated, and there was lots of mods for the c64 I could perform. Modding the systems is a BIG part of it for me. So that's another thing to keep in mind. Do you want to mod your own systems or just plug and play?
