1)WW1: The Renault: Cheap, easy to conceal, better manning requirements than the British rhomboids or even whippets. The M1 Abrams didn't exist then :)
2)WW2: Panther showed the way to the MBT. The Panther 2 would have been it. The Tu-80 didn't exist then :)
3)Korea: At the beginning, certainly the T-34. In the middle and end, the SHERMAN: Better power to weight ratio than later tanks, adequate firepower in an infantry war. The Tu-72 din't exist then, either :)
4)Vietnam: M-48 or the M-113. The Char B-1 would barely be able to take on PT-76's. :)
5) The Centurion. The panzer 111 would have been roadkill, even worse than the panzer 4's used by the Syrians.
6) Of course, the M1 Abrams. Nuff said. The A7V wouldn't stand a chance against the M1, but may be able to go toe to toe with the LeClerk as crewed by the French.