As smiths said, it's completely dependant on the individual WG, and largely on the individual punter for what they want...it's all completely subjective.
For what it's worth, the best looking WG I've punted was English, but the service didn't live up to her looks (it was pretty good though). The best service I've received, hands down, is from an EE (Polish in this case).
Not punted with a Romanian, for many reasons, but from the ads I've seen for them, judging them as "the cutest" doesn't ring true, but again this is entirely subjective.