Panzer VIII Maus
Panzerkampfwagen VIII Maus ("Mouse") was a German World War II super-heavy tank completed in late 1944. It is the heaviest fully enclosed armoured fighting vehicle ever built. Five were ordered, but only two hulls and one turret were completed before the testing grounds were captured by advancing Soviet military forces.
Tank destroyer
The closest the British came to developing an armored tank destroyer in the vein of the German Jagdpanzers or Soviet ISU series was the Churchill 3-inch Gun Carrier—a Churchill tank chassis with a boxy superstructure in place of the turret and mounting a 3-inch anti-aircraft gun. Although a number were ordered and fifty delivered in 1942, they were not put into service as the immediate threat passed. The design was rejected in favor of developing a 17 pounder armed Cromwell tank variant, ultimately leading to the Comet tank. The Tortoise "heavy assault tank", intended for use in breaking through fixed defensive lines, was well armoured and had a very powerful 32-pounder (94 mm) gun, but did not reach service use.