I'm not sure how you are defining "best" but if you mean winning the war via a colossal superiority in numbers over the enemy then you are correct. The late war variants like the Easy 8 and the Firefly were excellent matches against German armor but the earlier 1942-1943 variants were definitely inferior.
Exactly to what? In 1942 the Germans had, in quantity essentially two tanks. The PzKwIII and PzKwIV. The Panzer IVF2 wasn't put into production until very early in 1942 so it pretty much is a contemporary of the M4 Sherman. Prior models used the short barrel KwK 37 L/24 which, while capable of engaging tanks, wasn't designed for armored warfare. The M4 was better armored, at least as well armed, faster and the medium velocity M3 gun was not as powerful as the German L48 but it certainly did have the capability to penetrate any armor of the PzIII and PzIV. Add to this the fact that the Germans didn't have a great number of these up-gunned PzIVs and had to soldier on with earlier A-F1 model Panzer IVs. Every Sherman they met had the 75mm M3 gun. The Panzer III was pretty much outdated by the time of the introduction of the Sherman.
Now if you are talking about the Tiger, well, yeah the Tiger was bigger, more powerful and better protected. It also didn't hit the battlefield until the end of 1942 and less than 1500 were produced during the entire war. Comparing a medium tank like the Sherman to a heavy tank like the Tiger is like comparing the T-34 to the Tiger. In most scenarios the Tiger is going to win. The Americans, in contrast, created 52,000 Shermans by the end of 1943. The Tiger got a reputation as unstoppable and it continues to be reinforced to this day. As far as heavy tanks go, it is probably one of the few vehicles the Germans introduced during the war that lived up to expectations but it was never going to be a game changer. There were simply too few of them and they had to be spread too thin. If Germany could have made 10 thousand of them then there would have a much bigger impact. But all it ended up doing was looking fierce, fought and killed a few enemy tanks and then was slaughtered.
And as for the Panther, which didn't hit the battlefield in numbers until late 1943, it was the most overrated armored vehicle of the war. In theory it was a great tank but it never lived up to expectations. The 76mm and 17 pound equipped Shermans could engage these tanks with ease and unlike the Panther they didn't break down on the way to the battlefield. They never did get the transmission issues fixed on these tanks. And the Germans only made about six thousand of them. Again, even if it worked properly there simply wasn't enough of them. And they didn't work. More of them were found broken down than actually killed in the battlefield. If you capture a broken down tank that is the same as killing it.
This means that the vast majority of Shermans fought two models of German tank, the PzIII and increasingly the Panzer IV L48 gunned versions. Neither were better than the Sherman and in most cases were at a disadvantage one on one.
Every nation that used the Sherman loved it. The Russians loved the ease of maintenance, ergonomics and overall capability of the tank. Most actually preferred it to the T-34 because driving and riding in it wasn't a chore. The Brits credit it for winning the war in the Middle East. Again, reliable, rugged and superior to anything the Germans had at El Alamein. It was a great tank, even in 1942. The early variants obviously fell behind quickly as armored warfare evolved but that happens with all tanks. The PzIV was much different in 1944 than it was in 1938 and the T-34 had substantial, if incomplete improvements over the life of the production run. Personally, I think the Sherman was the best overall tank of the war.
