It's a close call between the Guardian and the Doug Skyraider, both of which had nearly the same max gross and wingspan, though the Guardian's was slightly larger in each category. Yet the Skyraider was a vastly more effective and far more numerous airplane; no Guardians were even operating by the time of the Vietnam war, when Skyraiders were still very active.
But let's widen the purview to include single-engine jets, for the fun of it. Nothing comes close to the Republic F-105, with the MiG-27 and Lockheed U2 trailing somewhat behind. NOTHING with one engine has ever carried as much as a Thud.