A GameBoy is the obvious answer here, but the IBM PC (the 5150, the original one) is another good choice if you want to have a lot of software to try it with.
I've been slowly working on my GB emulator for a couple of months now. There's a lot of resources around, which is good. Unfortunately there are also parts that are ambiguous and somewhat poorly documented, which has caused a lot of misery. I had to resort to looking up other people's implementations, and it's not a very satisfying way to develop.
My one advice would be not to skip any parts in the implementation ("I just skip the cpu timing so I can have something on the screen quickly"), as many things are interconnected and changing things becomes very difficult later on.
All in all it's a really fun project and at least I learned a ton. My original motivation was to learn Rust, but it turns out that was the easiest part.
