I would put down
1 buchanan and the run up to the civil war. In particular, he allowed floyd, his secretary of war to move US property to southern locations to be stolen.
2. He allowed Army officers and West Point cadets to resign to go join rebel forces.
3. FDR for supporting the Munich agreement to appease Hitler with Czechoslovakia.
4. FDR for failure to send a brigade to Europe upon the Germans reoccupation of the Rhineland.
5. FDR for the new deal, which kept the US in depression/recession for 8 more years. That weakened the US ability to intervene in Europe which could have prevented WWII or stopped it at the early stages.
,
Jimmy Carter, Iran debacle. The current ME situation is a direct result of that.
John Kennedy, Bay of Pigs. The communist government of Cuba could have been easily toppled. We ended up with the Cuban missile crisis because of his vacillation.
Harry Truman. Mismanagement of Korea. Truman micromanaged the war effort and recalled MacArthur. North Korea has nukes and a starving populace. Allowing MacArthur to finish the job would have created a unified Korea.
Two of these Presidents, Carter and Truman, lost their chance at reelection because of their blunders. If the theory of Cuban involvement in Kennedy's assassination is true, it cost him more.
You are right about 4 and 5, but I believe FDR was constrained by public opinion regarding Munich.