Heavy tank
The Char 2C was one of the largest tanks ever produced. At the start of World War II, France and the Soviet Union were the only countries to have inventories of heavy tanks, such as the Char B1, T-35, and KV-1. The Matilda II was designed under the British infantry tank concept which is similar to the heavy tank having thick armor and weighing more than their other tanks. It’s usually separate because they have less firepower with their cruiser tanks (comparable to mediums) at the time having the same main armament but more machine guns. Later war examples were the German Tiger I and II, as well as the Soviet IS series. Note that "heavy" versus "medium" is more a question of tactical roles than weight; the Panther, for example, was a "medium" tank that outweighed most Allied "heavy" tanks.
Valiant tank
The Valiant, under General Staff specification A38, began as a candidate for an assault tank, with the thickest armour on the lowest possible weight, for use in the Far East. It was to be similar in intention to the 40-ton A33 Excelsior although far lighter. As the Valiant managed the same 114 mm frontal armour with only 27 tons, it managed to achieve its primary goal, but only by making unacceptable compromises elsewhere. At a time when British tank design was already at its nadir, this "terrible price for the weight concession" led to what is probably the worst British tank of the war.