The Panzerkampfwagen Maus and a later "Buck von Rogers" Nazi supertank... either would be a good basis for a sci-fi super tank conversion project... >;)
Churchill's mole or "Nellie" N.L.E. Tractors. Winston Churchill sometimes referred to the machine as his mole and the prototype machine was dubbed Nellie. It was lightly armoured and carried no weapons. It was designed to advance upon an enemy position largely below ground level in a trench that it was itself excavating. On reaching the enemy's front line, it would serve as a ramp for the troops and possibly tanks following in its trench.
1/144 Landkreuzer P1500 'Monster' - Matuo Kasten
AMX ELC (Engin Leger de Combat) - Unusual and interesting french project of hybrid light tank and tank destroyer. Very light construction (only 6,7 -7 tons), 2 crewmembers (driver and commander, who was also gunner/loader) and unusual turret - 360 degrees turn was possible only when tank was stopped. Very thin armour and powerful 90 mm gun.