Don't get me started.

During the 1927 Mississippi Flood, Hoover became nationally known as a great humanitarian for his organization of aid to the flood victims. However, he forgot that he was a racist, and left African Americans to starve and suffer from disease in what looked mostly like internment camps. However, Hoover struck a deal with the African American political leadership to keep quiet in exchange for political influence if/when Hoover was elected president. However, Hoover (being a racist) broke his promise and that resulted in African American votes going to the Democrats ever after.
Hoover's response to the Great Depression was to plead big business to play nice with the desperate labor at its disposal, and to deport 500,000 Mexicans and Mexican Americans.
Finally, Hoover chose to combat U.S. veterans demonstrating in Washington D.C. with tanks.

Richard Nixon was elected President 8 years after announcing his retirement from politics. Although he softened US-Soviet relations, he also played an important role in losing the Vietnam War. He also supported the Pakistan army in their war against India, despite reports of Pakistani atrocities against Indian civilians.
However, he is probably most known for his involvement in the Watergate scandal, where some of his cronies were arrested for breaking & entering, and it was discovered that Nixon and the Republican party was teeming with corruption and dirty tricks...
All in all, he made a few bad decisions!