Kinda weird question, it depends on the girl. If you, like, after intense studies and evaluations come up with some sort of pattern - I still believe it would be inaccurate and probably just what you make of it.
Like for example say you have this idea that black chicks are great at sucking dick. When you finally have a black girl giving you a BJ, you will be thinking "damn, this black girl is great", thus kinda fulfilling your own prophecy
So in short: it doesn't matter. I guess I would have to say scandinavians because that's all I've ever tried :dunno: