I'll rank them:
1. Watergate: Literally destroyed Nixon's reputation. Now everyone sees him as pure evil and accuses him of crimes he DIDN'T commit. Plus, it created a new trope where every subsequent scandal is labeled as "_______-gate."
2. Court packing plan: Just how FDR thought this would pass constitutional muster is beyond me. It completely undermined the separation of powers, plus checks and balances.
3. Lewinsky scandal: While Clinton's crimes probably didn't merit impeachment, he still committed perjury and obstruction of justice (which, incidentally, is the real reason why he was impeached, not the affair itself.)
4. Teapot Dome: A scandal arguably worse than Watergate, but didn't leave as much of a bad taste in peoples' mouths.
5. Escalation of Vietnam War: Like the invasion of Iraq, was an honest mistake. But in hindsight, we should have been able to see pretty soon after Tet that Vietnam was a lost cause.
6. Bay of Pigs Invasion: Like the Iraq war and the Vietnam escalation, was an honest mistake. Not to mention that the US had a more direct interest in this mission.
7. Iran-Contra: Given the fact that the US government supported a number of authoritarian regimes during the Cold War simply by virtue of them being anti-communist (e.g. Jimenez and Pinochet), this was relatively minor scandal.
How do you feel about the GOP 2/3rds majority in 1866 eliminating 2 SCOTUS seats for explicitly political reasons? Mechanistically, it's equivalent to what FDR was trying to do in 1937.*
*FDR asked for 6 additional seats, but I highly doubt anyone who objected to his plan would have been fine with it if he only asked to add 2.