Oh I would say probably Paul in the days before he settled down with Linda, got married and began having babies.
John called Paul the sexual gladiator and I read once that even in his teenage years in Liverpool he was known as the Liverpool Bull. The estimates of his sexual conquests appear to be around six to seven hundred or more in a relatively short time period, George was second and he never really settled in to monogamy although he loved his wife. John was probably close to George and Ringo right there with him. It may be because Paul had that oh so innocent face combined with an aura of raw sensuality. But yeah Paul during his unmarried days was unmatched in the Beatles.