I put my money on Kennedy. Despite all the mystic and grace of Camelot, the Kennedy men surpassed Clinton in terms of promiscuity and infidelity. When JFK and RFK weren't sharing Marilyn Monroe like some brother's share a sock drawer, Kennedy was involved with Judith Campbell, who was the girlfriend of a Chicago mobster to stuffed the ballots in Kennedy's favor. He also had an affair with a 19-year-old who had initially come to the Whitehouse to interview his wife since she was an alumni of her school, and was given a prestigous internship-- even though she couldn't type.
Mind you these are only a few of the known mistresses...one could probably write an book, if not volumes, on all his escapades.