Speaking in terms of the U.S. only. It wasn't the most criminal, but Watergate is the most remembered, which is what also makes it the biggest. Look at how many scandals now, political are not, are named with the word "gate" at the end. Many younger people now have no clue what the Watergate scandal was actually about, but they all know the name.
In the United States, I believe that Watergate was indeed the biggest political scandal of the century. Never before had a sitting president resigned office because of the behavior and actions of his subordinates. Watergate was a scandal that nearly completely eroded public trust in their government, and we continue to feel its repercussions.
If we're limiting ourselves to just American politics then yes, Watergate was clearly the biggest with its only clear competitor being the Iran-Contra affair. However, if we extend beyond America, there were massive scandals in any number of countries over the century. It would be remiss to assume that only America's scandals are significant.
The Watergate scandal was certainly the biggest political scandal of the 20th century - mainly because it went all the way to the top. Nixon personally authorized the break in, and was at the center of the cover up. No other Presidents in the 20th century were so personally involved in such a blatant crime.
I believe that Watergate was the biggest political scandal of the 20th century. I believe this scandal was quite involved and we haven't had a scandal like it before or after. I also believe administrations are far more likely to get away with comparable scandals now because of the difference in technology.