I think Richard Nixon is the most corrupt US president ever. Richard Milhous Nixon was the 37th President of the United States, he is the only president to resign the office as a mean to avoid being impeached over the Watergate scandal. Watergate isn't the only scandal that tarred Nixon and his administration.
Vice President Agnew resigned a year before for reasons not related to Watergate: he was convicted on charges of bribery, tax evasion and money laundering during his tenure as governor of Maryland.
Nixon's CIA director, was convicted of conducting illegal surveillance on U.S. journalists and plotting assassinations in Chile.
Nixon's secretary had to resign after less than three months in office after altering a negative finding (while transcribing the tape) against a Nixon.