Well, Gotti, Jr. Gotti and Henry Hill are right up there. But my vote would go to Gaspar diGregorio, Joe Bonanno's second in command, who sold out to Stefano Magaddino, Bonanno's estranged cousin, during the Banana Wars of the mid-'60's. Not only was that an el-dumbo move, but diGregorio pulled a grade-A estupido earlier:
Bonanno had the good sense to stay away from the famous and ill-starred Apalachin, NY convention in 1957. He sent diGregorio in his place. But earlier that year, Bonanno had gone to Italy for a long vacation. Before he left, he gave his driver's license to diGregorio to be renewed. Gaspar stuck it in his wallet and forgot all about it. When the cops raided Apalachin and pinched diGregorio, they lifted his wallet--and Bonanno's license. That's how Bonanno, who wasn't even there, was "identified" as one of the top crime bosses in America--previously he'd been known as a dress manufacturer and a cheesemaker! Gaspar must have been the model for the joke about the guy who was drummed out of the Mafia for stupidity: they sent him to blow up a car, he burned his lips on the tailpipe.