M6A2E1
The Mutant was (and still is for some lucky few) one of the best American Tier 8 Premium heavy tanks you can get your hands on. This tank was indomitable from the front thanks to its absurd amount of frontal sloped armor, coming in at a whopping 240mm when accounting for the slope.
Even the turret offers 229 mm of frontal armor, making it an ideal candidate for going hull down in World of Tanks. In addition, this armor is one of the few that comes close to the T34, one of the best Premium tanks for farming credits in World of Tanks. The M6A2E1 was only available for users who preordered the Heavy Pre-Order Package, making it one of the rarest tanks in World of Tanks.
Update: The Mutant went on sale on April 1, 2017 for one day only, and we absolutely picked one up.