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.
You pick a console/device, and spend a long time understanding how it works at the low-level.
Writing an emulator for a NES, Gameboy, or similar is hard work but not impossible. Most people suggest you write an emulator/interpeter for the fantasy chip8 device as a good starting point - that can be done in a weekend.
Google "chip8 c++ tutorial", or "chip8 opcodes" if you're more interested in learning the details yourself - and if you're not interested in starting from the bottom up I'd imagine you'll struggle with a "real" device..
Write a CHIP-8 emulator. I followed this [1] language-agnostic guide. It covers all the details and oddities without actually giving you the code, so you still need to make the effort to get the result.
