My vote goes to the 17pdr. My father was a radio operator/gunner in a Sherman Firefly and he killed 2 Panthers and Tiger plus Mk3 and Mk4. The boxheads were good but they were scared of the 17pdr. I believe one got Wittman.
My late grandfather was an instructor in the army during WWII and got chance to fire a 17pdr. He rated it as the most amazing weapon he every got to play with. The other plus point for the gun was it was air transportable in a Hamilcar, thus 6th Airborne had a few on the ground on D-day along with their prime movers. I do recall seeing somewhere that the Firefly crew got into a habit of trying to camouflage the front end of the gun to make it look like a standard 76mm gun, as the boxheads were scared of the Firefly and they would be the first tank to be engaged by the Germans if they spotted one. There is some debate about what got Wittman. Most of his troop on the day he was killed were taken out by Firefly's on his right flank. However on his left flank were a troop of Canadian standard Shermans hidden behind a large wall and one of them got a close range shot in the aft quarter of Withman's Tiger with a 76mm which was the shot that killed him.