I think every two-, four-, six- and eight-engine airplane that carried--and flew on--a single test engine has so far been mentioned as a "world's heaviest single" candidate. Amusing, but irrelevant.
My own Top-10 list, culled from this thread and based on MTOW plus a "wingspan factor," is...
Boeing F-35
Republic F-105D
MiG-27
Lockheed U2
Tupolev Tu-91
Grumman AF Guardian
Tupolev Ant-25
Douglas Skyraider
Breguet Alize
Vickers Wellesley
Your mileage may vary, but that's my story and I'm stickin' to it.