It's clearly still the one that "popped our cherries," Watergate! Before Watergate, people basically just took what the government said for what it was worth, and had faith in what they said was the truth (aside from youth, who always questions authority). After that, we've had a larger group of people who always questions things.
Furthermore, it was and still is the ONLY scandal that directly brought down a Presidency, because not only did Nixon lose any support that he had from Democrats......he also lost most, if not all support that he had from his fellow Republicans as well, which is why he resigned.
Now some might say "Watergate.....meh!" However, I would say: "Watergate: burglary (The DNC Headquarters was one of the places even), wire tapping, obstruction of justice, tampering with evidence, destroying evidence, abuse of power, firing government officials who wouldn't sell out and tried to do their jobs (anyone remember the "Saturday Night Masacre....Hell, I was a baby & I've even heard of that), a President who literally thought he was above the law, a huge cover-up, paying people off to buy their silence, and a constitutional crisis as well.
There is a reason why, 30+ years after Nixon resigned, there was still a major buzz over who "Deep Throat" was, and it hadn't died down hardly a bit.....until that day late in his life....that Mark Felt outted himself as being in fact the mysterious informant known as "Deep Throat." It made household names out of Washington Post writers Woodward and Bernstein (really made their careers actually). Hell, to this day, when something happens that is a scandal......what do we say......(fill in the blank)Gate for a reason also!
Anyway, I don't see how anyone could argue that Watergate isn't still the biggest U.S. Presidential scandal.....even to this day, becasue it affected, and continues to affect so many people. It was so big. Hell it was a literally a criminal conspiracy. People actually went to jail. It directly brought down a President. Hell, President Nixon had to be pardoned by President Ford even to avoid the distinct possibility that he (Nixon) would have ended up in prison as well. And, possibly most importantly, it permanently stole the "innocence" of the American people......and we haven't recovered to this day. :2 cents: