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From my understanding the new Leopard 2A-6 from my undestanding has the edge based on technology, armour, and capability.
It is however not battle proven.
EDIT: The Leopard 2 has been in combat (just not tank to tank combat). The Canadians are using the A6 in Afghanistan.
Aren't the Challenger II the fastest tank in the world, over rough terrain?
The stipped down M1 without all of the extra armor was rumored to have hit 100 mph back in the 80's. I do not know if it is true or what all was removed. My guess would be that the turret was not on the hull. The old M1 would a have no trouble keeping up with the posted highways speeds.
DBII
The Armour School is working on drone tanks and vehicles with a two man crew. The Cmdr and driver will sit inside of the hull. This will reduce the target size greatly in a hull down position. The current battle computer allows the tank commnder the select the targets on a touch screen and put then in order of engagement. The computer will line up the shots and the gunner does his thing. Once the target has been destroied, the computer will set up the next shot. I do not think that it will much longer that the gunner will be needed. The biggest draw back will be that there will no longer be 4 men to do the maintainance. .