First, on the M4. The M4 was actually a decent tank. It wasnt designed to fight other tanks, because American doctrine at the time suggested that the tanks best role was supporting infantry and exploiting breakthroughs. It wasnt until '43 and onwards that the doctrine began to shift towards tanks which excelled at tank-on-tank combat. Further, the M4 was designed to go from America to Europe. That meant it was designed with shipping in mind, and certain weight, size, and shape requirements limited the tank. Finally, the American Army liked a gasoline engine. They were reliable, cleaner, and the fuel was a tad bit more common in the towns and cities of Western Europe. But, gasoline burns easily, much easier than diesel, and any tank that burns gasoline will itself burn if its fuel tanks are hit. So you have a tank which has a funny shape, thin armor, a gun thats not designed for tank-on-tank action, and itll catch fire almost every time the gas tank is hit. The M4 had some problems. But so did early models of the T-34, as well as the PzIV and Panther. But by Normandy, newer models of the M4 (like the Jumbo, British Firefly, and Easy 8) had been introduced which remedied the problems of the earlier Sherman models. In fact, the Easy 8 was an excellent tank killer, and performed well even through the 1950s. And as a platform, the Israelis made heavy use of the M4, upgunned to the 100mm Super Sherman, in the 1967 war. So the M4 got to be an excellent tank, for what it was.

The T-34 was much the same way. In '41, its armor design was revolutionary. With radically sloped sides, the T-34 was one of the best protected production series medium tanks in production at that point. And, the 76mm gun equipped on them was sufficient to kill most German tanks as well. But the T-34 had its own problems. Early models of the transmission tended to break down, and the tank was equipped with a small 2-man turret which caused all sorts of problems, from operating the main gun, to inter-tank coordination. But unlike the PZIV and the M4, the Soviets refused to make substantial changes to the T-34 when every tank was needed on the front lines. Thus, the T-34 remained relatively unchanged until the T-34-85 entered production in 1944. This is the penultimate version of the T-34-85, and radically improved the tank. The turret was made larger, and it instead incorporated 3 men, vs. the old 2. Plus, the 76 mm gun, which struggled against Panthers and Tigers, was upgraded to a larger 85mm gun, which was still competitive against most enemy vehicles. While the armor of the tank wasnt really increased, the tank remained fast, and with the better turret and gun, it could really compete with the medium tanks the Germans were building at the same time. And like the M4, the T-34-85 went on to serve as the frontline tank in places which ranged from Germany, to Egypt. T-34s were used in the invasion of Cyprus in 1974, and they were used in the Balkans throughout the 90s as second-line tanks. As such, the T-34 has earned the distinction of being (as far as I am aware) the longest serving armored vehicle in modern history, which is a big reason why it has earned such a fanbase.

Generally, the M4 and the T-34 are regarded as some of the best tanks ever produced, because most observers take production numbers into account. German industry was simply never large enough to produce weapons on the scale that either the US or the USSR could. And combined, both nations built over 100,000 of their legendary medium tanks, plus thousands more heavy and light tanks. Germany, on the other hand, managed perhaps only 25,000. This is why the Germans placed such an emphasis on quality, rather than quantity. To maintain a competitive force, German tankers had to account for sometimes 2, 3, 4, or more enemy tanks for every one German tank that was knocked out. So the Germans tried to design (and redesign) their tanks to survive longer and dish out more damage than their enemies could. Tough tanks like the Tiger, King Tiger, and Panther all tried to maximize Germany's limited resources by focusing on quality.

And production numbers aside, I would argue that the Germans succeeded in the Panther design. If we were to just compare tanks solely on their performance characteristics, late-war Panthers were tougher, they had an amazing gun, they were reliable, and they they were fast. Further, the Panther was innovative because it created a new concept in medium tank design, and is often regarded as being the first "Main Battle Tank" ever built. As opposed to American and Soviet thinking, even late into the war, which clung to the old concepts of strictly defined roles, the Panther could do it all. With its thick, sloped armor, it performed well in either the attack or the defense, while the powerful main gun made it competitive against even the heaviest Soviet tanks. Post-War thinkers saw the Panther as an excellent blend of the three main characteristics of an MBT: Mobility, Toughness, and Firepower. And many tanks have followed the Panther's general design, layout, and blend of those three characteristics.

But like Ive pointed out, German industry could never match the numbers put up by Soviet and American industries. So its hard to answer the question of "what was the best tank then/ever?" (I would also argue that that, itself, is a pretty pointless question prone to "fanboy-ism" and major bias). But any answer to the question has to be quantified, both by the characteristics with which you select that choice, as well as what categories of tanks youre considering.

In my personal opinion the Panther was the best tank of the war. Plus it looked cool.