For at least 35 years, the U.S. embargo on diplomatic or commercial dealings with Cuba has been the single stupidest aspect of U.S. foreign policy.
Not the most destructive: that title would go to the decision to invade Iraq, plus the ongoing ramifications of the age of torture, open-ended war, and the security/surveillance state.
But the Cuba policy has been the stupidest, because there have been absolutely zero rational arguments for its strategic wisdom or tactical effectiveness. Jeffrey Goldberg, who has traveled in Cuba and interviewed Castro, more tactfully calls it "ridiculous." In my impetuous youth a few years ago, I called it not the stupidest part of U.S. policy but the "most idiotic." Take your pick.
I choose "at least 35 years" as the demarcation point for undeniable irrationality because that is when the U.S. fully normalized its relations with mainland China. If successive Republican and Democratic administrations could see the merit of trying to engage (rather than exclude) a one-party repressive communist-run state, even when that state had four times as many people as the U.S. did, and is nuclear-armed, and is a regional rival of several U.S. allies, how much more obvious is the case for a tiny little island practically within eyesight of the American mainland and certain to fall under the sway of U.S. cultural and economic influence if given a chance?