The Landkreuzer Ratte is an April fools joke made on January 4th 2012. It is well known for its huge size, and the ability to one-shot a Maus without shooting the ammo rack of course. But then again, this was only a joke so they never actually released this tank in the game. They did make a video of its gameplay though, so if you like to see how it looked in-game and see how huge it actually is, go ahead and click here.
If this were released into the game, it would become the most powerful, yet limited tank.
Size is the most obvious problem. Most cities and towns would not allow this monster to come through urban areas. Also, artillery will most likely have a field day on this large beast.
This monster arms (According to Wikipedia....) a naval-based 280mm gun, being from the Scharnhorst heavy cruiser class. Its secondary battery includes a 12.8cm gun from the Maus and 8 Flak 38s (Not from the Pz.Sfl. IVc). The main cannons would have most likely hold the capability to one-shot every tank in the game.
The armor this monster can boast is 360mm at the front, but 150mm at most areas besides the front, penetrable by most tanks. Only the Jagdpanzer E 100 can punch through (With its HEAT shells).
For its size and weight, it's incredibly mobile, capable of reaching a medium speed of 40 km/h thanks to its powerful marine engines that pump out more than over 16000 HP.