Well, that's something like asking "what is blue?"

There isn't really a -set- criterion for a main battle tank. The idea as we know it today may be traced back to Montgomery's proposal for a "capitol tank", later reformed into a "universal" tank, but an "MBT" as defined is a bit nebulous. It's said that an MBT replaced the three previous criteria of light, medium and heavy tanks, but even that is another example of the fun of nomenclature. Most people accept Tiger as a heavy tank, but was Churchill, coming in at near the same weight, despite being classified as an infantry tank? How about assault tank like T14? The Germans would probably have qualified it as a heavy tank, yet the Americans distinguished between the two. Further, the three types have not been entirely replaced: Light tanks are still a thing.

The perfect example of the problem of determining the first MBT is the fact that of the three commonly considered progenitors, none of them were designed as MBT. They are Panther, T-54 and Centurion. The first two were designed as medium tanks, and the Centurion was designed as a cruiser. (The British splitting their designation by operational role rather than weight). Panther's career, of course, was cut short after the war, but T-54 and Centurion hung around in service long after the concept of the MBT was identified, and, more to the point, long after countries stopped dividing their tanks into light, medium and heavy. Panther didn't last that long, but that's not the fault of the design. Well, other than its fault for losing, I guess.

I asked a panel of experts once as to their opinion of 'first MBT', and the only one to hazard a response was Steve Zaloga, with Panther. In that case, you're talking about a tank which is heavy enough, mobile enough, and powerful enough to perform the wide range of roles a tank might be expected to perform, just like Cent and T-54. As the one which came first, by that criterion, Panther can arguably get a nod.

Now, there's the point that Centurion and T-54 were still contemporaneous with heavy tanks, Conqueror and T-10 (And obviously Panther with Tiger). What Chieftain did do was bring along a gun equivalent to the heavy gun tank and, as such, would replace both Centurion and Conqueror. For those who argue that replacing the heavy tank is a requirement for an MBT, this is the nod for Chieftain. I personally disagree. That the British had such a heavy tank in service does not change the characteristics of the smaller tank. It would be like saying that because the Soviets had T-10s on the books when T-64 or T-72 came out (Technically they still had IS until well into the T-80 era), T-64 and T-72 were not MBTs and few would make that argument.

There are a whole bunch of heavy tracked vehicles out there such as support tanks (BMP-T), mobile gun systems, tank destroyers, and so on, but as far as true tanks go, only light and MBT, with an honorable exception to the Argentinian TAM which is a bit heavy for the light category, without being as tough as an MBT. Plus I would argue that there could be a consideration for the wheeled Japanese Type 16 MCV as a light tank given its role.

There is no TD variant of the M1A1/M1A2. (It can be argued that M1A1 itself was a TD, as it only fired armor-defeating rounds)