Gotta go with the 155MM Long Tom. The SP version. While there were other guns out there that had larger diameters or had technical points of superiority, in terms of number, mobility, size and effectiveness.
Read a story of an attack on the West Wall in 1944 where an infantry unit was pinned down. A Long Tom cranked up (evidently, it was wandering around the battlefield looking for it's unit) and asked if it could help. "Sure" said the infantry. And the gun set up and proceeded to throw 155MM rounds at bunkers and whatnot until the opposition was reduced. Not the kind of information that shows up in statistics but that use of Artillery is hugely important.