Oh I can answer that.
The best tank is the most durable one, the most survivable. Your big guns and big engines are no good when your tank gets flamed by an RPG and the photos of the burning wreckage gets stamped on every newspaper, the story gets broadcasted on all news channels and the images are displayed night after night untill the support for whatever war drops just enough to pull back.
So, forget about Main Battle Tanks, you want a Massive Brick Titan:
Or something along that line, with more machine-guns to repell clever bastards climbing on the thing to set up a shaped-charge.