There really are no rules to define a scandal.  Normally anything that is illegal and involves a politician can be termed a political scandal. There are also no lines to delineate between a major scandal and a minor one – that is normally defined by the public and the media!
Here are ten  of the most known ones:

1. Watergate

This scandal involving a burglary in the Democratic National Headquarters in Washington in 1972, spiraled into a major conspiracy originating from the White House. It led to the conviction of many of Richard Nixon’s staff members, and ultimately to his resignation. He was the only President to have ever resigned from the White House.

2. Iran Contra Scandal

This scandal was revealed in 1986, during Ronald Reagan’s term. Senior US officials had facilitated the sale of arms to Iran, despite an embargo, to secure the release of American hostages and to secretly fund the Nicaraguan rebels. Following an investigation, eleven officials were convicted.

3. Monica Lewinsky Scandal

Monica Lewinsky, an intern in the White House, had intimate relations with Bill Clinton, the President. She confided about the affair to her co-worker Linda Tripp, who tape-recorded the conversations, and then released them to Independent Counsel Kenneth Starr. Clinton became the second President to face an impeachment trial, however he was acquitted by the Senate.

4. Bofors Scandal

This scandal seems to be never ending – since it first raised its head in India in the 1980s and has repercussions even today. It began with the accusations of high kickbacks in political and administrative circles from Bofors AB for the purchase of Howitzers. It led to the defeat of Rajiv Gandhi’s  Congress party in 1989. No charges have been proven yet, but the innuendoes continue to fly even now.

5. Spiro Agnew

Very few people know about this man – but that was the case even when he was chosen to be Richard Nixon’s Vice President. He had to resign from his post in 1967 due to his ‘no-contest’ plea on bribery charges. He is the only Vice-President who had to resign due to criminal charges.