Deadliest tank of all time... ha ha ha no bias here!
Tanks are built according to their doctrine and expected environment. You build it for the job. Russians have (or had) a different set of needs to the Americans and built very different tanks to meet them. They would find an M1A1 inferrior for mass attacks, their god-aweful Russian bogs, extreme winter cold and mass production, not to mention distances they have to travel. You build it to suit the job. There is no best tank.
That said, the M1A1 is pretty awesome. But so's a Merkava, or a Leopard-2, or a Challenger-2, or a T80.