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The great disaster at Arausio is mentioned above, and Quintus Servilius Caepio surely deserves consideration as worst commander. Apparently, because they were political enemies, Caepio willfully refused to cooperate with Gnaeus Mallius Maximus who was consul in 105 and legally his superior officer (Caepio was only proconsul); allowing both armies to be destroyed in detail by the Cimbri and Teutones. Caepio was recalled, stripped of his proconsular imperium (I believe the only Roman commander so punished) and convicted by the people in the sensational "Gold of Tolosa" trial. He was stripped of his priesthood, exiled and his property confiscated to the ruination of his family according to Livy.

Varus and Caepio are somewhat tied in my book. Arausio was a dreadful defeat - eighty thousand Roman soldiers killed or captured because two pigheaded commanders refused to work together. But the Battle of Teutonburg was more consequential in the long run.