Andrew Johnson
He became president in 1865 after Abraham Lincoln was killed.  Congress was then run by Republicans, and after Lincoln's assassination, they wanted stricter terms than Johnson did for the Reconstruction of the Southern states that had rebelled. Congress was also more friendlier to African Americans who had recently been slaves, and many Republicans wanted them to vote and be given land. Johnson, who was a Democrat, thought it would hurt white people in the South and was strongly opposed to these policies. As a result, he vetoed 29 bills passed by Congress, and is the president to have the most vetoes overridden. This can happen if Congress passes the law a second time with a 2/3-majority, meaning twice as many people supports the law as opposes it. If this happens the veto fails and the law passes anyway. It is very unusual but happened 15 times to Johnson, which is a record.