I personally wrote a GB emulator, while it was a lot of work is was certainly doable compared to other options and the result is pretty satisfying. I know other

I personally wrote a GB emulator, while it was a lot of work is was certainly doable compared to other options and the result is pretty satisfying. I know others have started with the NES and had similar success, so IMO either of those are not bad. There's plenty of information out there for emulating those systems since lots of others have written emulators for them, so you should be able to find a lot to start with without too much trouble. If you find those systems to be a bit too much I've heard that CHIP-8 is even simpler. At some point being too simple doesn't teach you all that much, but it might help with the basic idea if you're really lost and also take less time if you're not looking for a long project.