Genesis: Unknown, but avoid older versions of Gens, cause their sound is terrible.
PS: Emulating this system is still a decade behind. Nothing is accurate.
N64: This is even worse than the PS, and is more than a decade behind.
GB/C/A: Probably one of the VBA forks, but I have no idea. Hasn't been much word on this in a while.
DS: Still pretty new, so it's unfair to expect anything to be that accurate yet. Patience.
Saturn: From what I hear, nothing has been worked on in at least 4 years. Unknown what is accurate, if anything.