I think that you will find variance on the answer to this question. In my mind, I think that each possessed a rather disturbing side to the modern conception of the President's office. Each one features an example of the President or members of his candidate acting outside the boundaries or norms of the office. I would say that the most dangerous would have been Watergate because it put forth the fundamental questions that would guide the other two investigations: What did the President know? When did he know it? These two issues first arose in Watergate and became the guiding questions that Reagan and Clinton had to answer. I think that Iran- Contra has received some of a pass over time because of the overall Regan Presidency, which has been perceived as a mostly positive experience. The Clinton scandal was one that ended up being more tabloid and while its symbolic scar on the president's office was present, it was not as nearly as Constitutionally challenging as Watergate.
Public vs. private is the major distinction of the three political scandals. Only one of the scandals, the Iran-Contra affair, involved scandal on an international scale. However, each scandal made an impact on U.S. history.
Watergate has been called the greatest political scandal of all time. It was a scandal of epic proportions, even though it only involved Americans directly. Pres. Nixon, who held the most power and influence in the U.S., directed and funded (using taxpayer money) a break-in. All of the evidence pointed to Nixon, including wire taps he had installed in his own office. After being found guilty of the actual and related crimes, Pres. Nixon was forced into a humiliating, public resignation.
President Clinton faced impeachment and removal from office after it became obvious that he used carefully-worded language to lie about an affair. During that time, many people stated that Clinton's lies hurt no one but his family. The belief was that the president's behavior was his own business. However, the affair did tarnish the world had of the U.S; jokes about the president were common.
Government officials normally refuse to negotiate with individuals who hold captives however the Iran-Contra affair was a case of indirect negotiation. The Iran-Contra affair had an impact on the U.S. international standing. Not only was diversion of government funds and cover-up exposed, but this affair was on an international scale. The situation began innocently enough with an attempt to help Iran, but the effort fell through. No evidence of the unethical act points directly to President Reagan.
Pres. Nixon's offense is the most offensive because he willfully committed a crime and then covered it up.
Great question. In all three of these examples, impeachment proceedings were at least talked about, and in two of those cases, followed through to the Senate trial phase. So I guess you can say they share the fact that some people in government at the time of the incidents believed that impeachable offenses had occurred.
I agree that Watergate was the worst of the three, although I also tend to weigh the fact that Nixon did a number of other unconstitutional things that are part of a pattern that make Watergate worse. The bottom line is he was trying to rig a Presidential election he subsequently won, and that is certainly justifiable to then want him removed from that office.
Iran-Contra involved a covert operation. And while the President can and does order such operations, this one involved two questionable things: 1) ignoring the stated, voted upon will of Congress regarding Contra funding, and 2) committing illegal transactions by selling weapons to Iran, violating the embargo.
Clinton's scandal with perjury - lying under oath about the Monica Lewinsky affair - seems the least qualified as an impeachable offense. It was stupid, to be sure, and Clinton used laughably bad judgement for a professional politician in that case. But in the end, he lied about doing something that wasn't illegal, and his impeachment seemed heavily due to politics, Republicans vs. Democrats.
This is, of course, a matter of opinion. My opinion is that Watergate was way worse than either of the other two.
The reason I say this is that Watergate involved an actual cover up ordered by the President. This is extremely serious in that President Nixon was ordering people to use their governmental powers to obstruct justice.
I think that this is more serious than what Clinton did. Clinton was lying, but he was not using the power of the government to try to cover up what he had done. I also think that his basic "crime" was much less important since it did not involve trying to subvert the process of democracy.
As far as Iran-Contra goes, the part that Reagan clearly knew about (the idea of supporting the Contras after Congress said not to) is not that bothersome to me because the branches of government try to maneuver to get around each other when they feel the need to do so.