Most people start with chip8 from what I hear (or is it see since were on forum? )
But there is easier systems t emulate. Such as anything that has the 8080... Or do a single arcade emulator of space invaders...
Check out this page and look at the categories near the top... Those are probably easiest.
Maybe someone should compile a FAQ for programming forum, since this question seem to occure quite often? I agree that Chip8 is one of the easiest things to emulate.