My vote goes to the Panzerfaust handheld anti-tank weapon.
In the Battle of Normandy, only 6% of British tank losses were from Panzerfaust fire, despite the close-range combat in the Bocage landscape. . . . In urban combat in the late war in eastern Germany about 70% of tanks destroyed were hit by Panzerfauste or Panzerschrecks.
During the battle of Normandy, only the Panzerfaust 30 (30 meter range) was available. In September of '44, Germany began producing the Panzerfaust 60 (the most common version). Later versions included the Panzerfaust 100, the Panzerfaust 150, and even the Panzerfaust 250. (Though the latter two were not produced in quantity.) As a standard of comparison, the Bazooka could penetrate 76 mm of armor. The Panzerfaust 60 could penetrate 200 mm of armor, and the Panzerfaust 100 could penetrate 220 mm of armor. In a letter dated from 1944, General Patton advised against using the Bazooka at a range of above 30 yards (about 30 meters). That’s the same range as the earliest Panzerfaust (Panzerfaust 30).
The US 82nd Airborne Division captured some Panzerfauste in the Sicilian campaign, and later during the fighting in Normandy. Finding them [Panzerfauste 30s] more effective than their own Bazookas, they held onto them and used them during the later stages of the French campaign and even dropped with them into the Netherlands during the Market-Garden campaign. They captured an ammunition dump of Panzerfauste near Nijmegen, and used them through the Ardennes Offensive to the end of the war.[12]
The Bazooka and Panzerfaust 60 each weighed 13 pounds (6 kg). Both were relatively simple, easy-to-manufacture weapons. Most versions of the Panzerfaust had a disposable tube. But the Panzerfaust 150 (deployed in limited numbers near the end of the war) had a tube good for about 10 shots. Its projectiles had double the velocity of the Panzerfaust 60’s; resulting in significantly improved armor penetration (and from a much longer range). Note that even the Panzerfaust 60 already had double the range and nearly triple the armor penetration of a Bazooka. Manufacture of the Panzerfaust 150 began in March of 1945; too late to affect the outcome of the war. However, as Germany changed from the Panzerfaust 30 (Normandy invasion) to the Panzerfaust 60, and later to the Panzerfaust 100, the percentage of Germany’s tank kills scored by Panzerfausts steadily increased.