The Panzerfaust for impact on the battlefield, alone. No other weapon caused such a dramatic rethink in armoured conflict. It was not the fact that this RPG was the best because it wasn't, what made it have such a large impact was the fact that it was one shot, one kill then drop it and run off. Disposable weaponary, easily mass-produced and deadly.
Soviet tank crews were forced to formulate ideas to cover every single part of the battlefield because German tank hunters could be anywhere. In Berlin, 1945, when the IS-2s started rolling down the German streets it was always one on the left covering the right, one on the right covering the left and one at the back covering both and ready to move up. Why? Because they knew that anyone could be in a building, cellar, under rubble anywhere with a Panzerfaust. Shoot it, and run off into the streets.
The U.S Bazooka was a heavy piece of machinery, and often required two people to work it efficiently. When an enemy armoured column is rolling through the streets, you want to be fast and mobile to hit and not be hit the Panzerfaust gave you this ability.
It's always hard when you only have one to choose because both the Mg-42 and Stg. 44 were brilliant pieces of weaponary and certainly they were revolutionary.
I would like to point out though that some 500,000 Stg. 44s were used by the Eastern German Army after World War 2. The Russians would have certainly seen them...
On the tank issue, going back to basics, the T-34/85 was not as good as the Panther Ausf G. The sloped armour on the Panther gave it more frontal protection than the Tiger Is slab-sided armour. After the intial clutch problems were sorted on the Ausf A the Panther proved to be reliable, it was fast and mobile. The 6th Coldstream Guards captured a Panther Ausf G that had been abandoned in full working order in the Ardennes Offensive, nicknamed it Cuckoo. They remark on the Panthers ability to hold the road in icy conditions when their tanks (most likely Churchills) were slipping and sliding all over.
The cannon on the Panther was superior to the T-34/85, the optics, the radio, the armour, the weapon on the Panther were all superior to that of the T-34/85. In fact, the comparable Russian tanks to the Panther would be the IS-1 and IS-2. The IS-2 could destroy the Panther at 1000m, the Panther could return the favour at 800m. Their armour was almost equal, the Panther was superior in equipment (Radio and optics) so had more tactical ability on top of more actual chance of striking the target.
The Panzerkampfwagen V Ausf G 'Panther' was probably the best all round tank of World War 2 but was unfortunately over-shadowed by Germanys dying war and the constant madness in Hitler to build bigger and bigger tanks.
Late in the war, everyone with a bit of sense realises that Germany was running low on...well...everything. This affected its tanks, of course, but it doesn't make the tank design any worse.
On the King Tiger, not one single King Tiger was destroyed through a frontal penetration. There is absolutely no evidence that this could have been achieved, in battle conditions. The only pictures of a King Tiger destroyed in such a way are from months after the war, when Russians were testing weapons on a captured Tiger II. And we all know, that sat in a field with a A-19 122mm cannon being rolled up at optimum range isn't battle conditions. The King Tiger also had, in that nice shiney slanted turret, the most powerful tank cannon of the war the KwK43 L/71 and the presence of the Tiger Royal alone caused fear.
Now, I'll leave it there. Feel free to state how unreliable and heavy the King Tiger was. Throw in a little bit about bridges, and the fact that most couldn't support it. And then, hopefully, state the malfunctions on the Panther Ausf A that caused it to set itself alight. And it's poor performance during Kursk with these malfunctions...