Most fun retro console/home computer to develop for

If you intend (or would at least like) for other people to play/try out/use what you make, consider going with a Commodore 64 instead of a 128; for people with physical hardware, the 64 is far more popular and will have a wider base of users.

Of course the 64 is a bit more limited than the 128, but handling that is part of the fun of retrocomputing. "It's not the lines that make playing Tetris interesting - it's the walls."