[ant6n]

Game Boy, Game Boy Color, Game Boy Advance.

As a programmer, these are machines that you can understand in their entirety down to the metal (I don't mean the hardware internally, just everything that's visible to the programmer), because they are relatively simple. It's also hardware that many grew up with and can relate to. They're well documented and ready to develop for, including very good emulators and debuggers. They're cheap, or you can just use an emulator to play around with them.

GB original is fun because it's so limited, and it's like a puzzle to figure out what you can do with it (including making graphics that prayers can actually recognize). Writing asm for it is like an adversarial puzzle.

GBA is based on ARM so it is a relative of modern cell phones, but running only at 16Mhz. You can still do a lot with it, because it's still pretty powerful (e.g. it runs Doom). It also has a very powerful 2d graphics chip that's still easy to work with and try to figure out effects for. Writing asm on it is a joy.