I still stand by my original post in this thread.
Altirra is regarded as the most accurate and controls are annoying to setup, Kat5200 is extremely good and easy to setup while Mame is ok it has issues like broken controls in certain game. I cannot comment on Rainbow as I have never used it or even scene it talked about before. I have used the 800 core in Retroarch and it is fine and functional from my use but that is all I can say.
My personal favourite of the 4 I have experience with is Kat5200, easy to setup and supports some basic 1 pass shaders from Retroarch (if you care about CRT effects).