At the risk of ridicule, I’d say the B-17. This was in many respects a great plane, but measured against its designers’ claims, it was a dud. First, the whole point of the Flying Fortress is captured by the name, but, in fact, B-17s could not fly unescorted bombing runs against Axis fighters and were lost in horrifying numbers when they tried (at Schweinfurt, etc.). Second, the B-17 was designed to achieve pinpoint accuracy and couldn’t. So by comparing the end product with the plane specified by the Air Force and described by the designers and manufacturer, we find the B-17 profoundly overrated.
Some much-scorned World War II fighters—the Brewster Buffalo, the P-39, and the P-40. All were used with effect and indeed enthusiasm by the foreigners who were very grateful to have them; the Royal Indian Air Force used the Buffalos against the Japanese, and the Finns got good service out of them against the Russians in both the Winter War and the early part of World War II. The Buffalo had remarkable range for an early World War II fighter, better than twice the operational radius of a 1940 Messerschmitt Bfl09, Spitfire, or Hurricane. The Russians loved the P-39s and P-40s, which were good multipurpose aircraft; they used them both as ground-attack craft and as low-altitude fighters. Chennault had excellent results with P-40s against Zeros, as did other U.S. pilots after developing tactics to cope with the Japanese. The P-40 did not perform as well at higher altitudes, but most German and almost all Russian fighters had similar troubles, and a lot of air combat took place close to the ground.