M551 Sheridan. Great concept that should have been developed further.
German "Elefant" tank destroyers on the assembly line. These were a Schwerer Panzerjäger ("heavy tank-hunter") of the Wehrmacht during World War II, built in small numbers in 1943 under the name "Ferdinand" after its designer, Ferdinand Porsche, using tank hulls that had been produced for the Tiger I tank requirement, but which were rejected in favor of the competing Henschel design.
A heavy tank hunter "Ferdinand" acting as fire brigade on East-front, near Nikopol (Ukraine), late '43.