Tomo, Best/Worst is always opinion based. Is a knife better/superior to a gun? Depends.
Production of the Sherman was favored by the commander of the Armored Ground Forces, Gen. Lesley J. McNair over the heavier M26 Pershing, which resulted in the latter being deployed too late to play any significant role in the war. McNair, an artilleryman, championed the tank destroyer within the U.S. Armored Forces. Tanks were to support the infantry, exploit breakthroughs, and avoid tank-to-tank battles. Enemy tanks were to be engaged by the tank destroyer force, composed of a mix self-propelled tank destroyers and towed antitank guns. The tank destroyers were supposed to be faster and carry a more powerful anti-tank gun than tanks since armor was sacrificed for speed. The tank destroyer doctrine played a large role in the lack of urgency in improving the firepower of the M4 Sherman, as the emphasis was on its role as infantry support. McNair opposed development of the M26 and other proposed heavy tanks during the crucial period of 1943 because he saw no "battle need" for them. In mid-1943, Lt. General Devers, commander of U.S. forces in the European Theater of Operations asked for 250 M26s for use in the invasion of France. McNair refused. Devers appealed to General George Marshall, the Army Chief of Staff. Marshall summarily ordered the tanks to be provided as soon as they could be produced. Soon after the Normandy invasion, General Dwight D. Eisenhower urgently requested the M26 Pershing, but McNair's continued opposition delayed production. General Marshall intervened again and the tanks were eventually brought into production. However, only a few saw combat and were too late to have any effect on the battlefield.
The M4A1 Sherman first saw combat at the Second Battle of El Alamein in October 1942 with the British 8th Army. The first U.S. Shermans in battle were M4A1s in Operation Torch the next month. At this time, Shermans successfully engaged German Panzer IIIs with long barreled 50 mm L/60 guns, and Panzer IVs with short barreled 75 mm L/24 guns. The M4 and M4A1 were the main types in U.S. units until late 1944, when the Army began replacing them with the preferred M4A3 with its more powerful 500 hp engine
Encounters with a company of Tiger Is, with their heavier armor and 88 mm L/56 guns, in Tunisia were a disaster for the Shermans, however, the fearsome quality of a few German heavy tanks and their crews could sometimes be overcome by the quantity and mobility of the Shermans, supported by artillery and airpower, but at a great cost in U.S. tanks and crewmen. By June 1944, the Panzer IV had been up-gunned with a 75 mm L/48 weapon, and the 75 mm Shermans were outgunned on a regular basis. As a result, the M4A1, was upguned to the 76 mm gun in July 1944, closely followed by the M4A3
In typical Army fashion the new 76mm and 90 mm anti-tank guns were rejected by McNair as "unnecessary".
In 1943, most German AFVs mounted a 7.5 cm KwK 40. As a result, even weakly armored light German tank destroyers such as the Marder III, which was meant to be a stop-gap measure to fight Soviet tanks in 1942, could destroy Shermans from a distance. The U.S. 76 mm proved comparable in penetrating power to the 7.5 cm KwK 40, however transfer of the upgunned tanks to the front started slowly, and most tanks still had M3's, even by the time of Operation Cobra.
The bigger 76 mm gun could penetrate roughly 88 mm of armor at 1000 m. This was enough to reliably penetrate a PzIV's glacis. However, the 76 mm was not powerful enough against the frontal armor of a Panther. Due to its angle, the Panther's glacis gave it an effective thickness of 140 mm. Therefore to effectively engage a Panther the Shermans had to get relatively close. Additionally the low-flash powder of the Panther made it harder to spot. While firing from range, the Sherman's high flash powder made their shots easy to spot. The Sherman's gun sights were fixed magnification as compared to the German's multiple magnification settings with added anti-glare filter. U.S. tank units who were engaged at range from German defensive positions sometimes took 50% casualties before even spotting where the fire was coming from. The British-developed Sherman Firefly was an M4 re-gunned with their QF 17 pounder anti-tank gun. The 17 pounder was a 76 mm gun and had a 55 caliber barrel, but introduced a much bigger charge which allowed it to penetrate 140 mm sloped at 30 degrees at 100 m and 120 mm at 1000 m. This gun allowed the Firefly a slight firepower advantage over the Panther, however the muzzle flash due to unburnt powder from the increased charge left crews momentarily blinded after firing. In late 1943, the British offered the 17 pounder to the U.S. Army for use in their M4 tanks since the 17 pounder could be mounted in a standard M4 turret while the U.S. 90 mm gun would need a new turret. General Devers insisted on comparison tests between the 17 pounder and the U.S. 90 mm gun. The tests seemed to show that the 90 mm gun was equal to or better than the 17 pounder. By then, production of the 76 mm M4 and the 90 mm M36 tank destroyer were both underway and U.S. Army lost all interest in the 17 pounder.
In terms of mobility the M4 was criticized by its crews for inability to pivot turn, limiting its usefulness against pivot-turning Panthers. This deficiency was partially compensated by the faster traverse of its turret. The U.S. Army restricted the Sherman's height, width, and weight so that it could be transported via typical bridges, roads, and railroads. This aided strategic, logistical, and tactical flexibility. The Sherman's speed and cross country performance was indeed superior to the first generation German tanks such as the PzKfpw. III IV, actual comparative testing with the second generation German tanks (Panther Tiger) conducted by the Germans at their Kummersdorf testing facility as well as by the US 2nd Armored Division, proved otherwise.
Lieutenant Colonel Wilson M. Hawkins of the 2nd AD wrote:
"It has been claimed that our tank is the more maneuverable. In recent tests we put a captured German Mark V [Panther] against all models of our own. The German tank was the faster, both across country and on the highway and could make sharper turns. It was also the better hill climber"
Technical Sergeant Willard D. May of the 2nd AD wrote:
"I have taken instructions on the Mark V [Panther] and have found, first, it is easily as maneuverable as the Sherman; second the flotation exceeds that of the Sherman"
Staff Sergeant and Tank Platoon Sergeant Charles A Carden reports:
"The Mark V [Panther] and IV [Tiger] in my opinion have more maneuverability and certainly more flotation. I have seen in many cases where the Mark V and VI tanks could maneuver nicely over ground where the M4 would bog down. On one occasion I saw at least 10 Royal Tigers [Tiger B] make a counter attack against us over ground that for us was nearly impassible"
The Sherman was one of the first widely produced tanks to feature a gyroscopic stabilized gun and sight. The stabilization was only in the vertical plane, as the mechanism could not slew the turret. The utility of the stabilization was debatable and some operators disabled the stabilizer
The Russian T-34 is often credited for introducing sloped armor in a production tank however one can easily see that the Sherman's upper hull was angled at 56 degrees, while the lower half of the hull was curved. The steel frontal turret armor of the M4 ranged from 64–76 mm. The M4's gun mantlet was also protected by 76 mm of armor sloped at 30 degrees. The turret side armor was 50 mm at 5-degree angle while the rear was 64 mm at a 90-degree angle and the turret roof was 25 mm thick. The hull front sported 51 mm armor. Perhaps the most telling report on the armor of the Sherman compared to the Panther can be described by statements used in a report to General Eisenhower at SHAEF: "I have actually seen ricochets go through an M4 at 3000 yards. I have seen HEAT fired from a 105mm Howitzer at a Panther at 400 yards. The track was hit and damaged, and a direct hit on the turret only chipped the paint."
The worst fault of all is what happened after the Sherman's inadequate armour was penetrated. The Sherman quickly gained grim nicknames like "Tommycooker" by the Germans. While the British took to calling it the "Ronson", the cigarette lighter which had the slogan "Lights up the first time, every time!" Polish tankers referred to it as "The Burning Grave". Research conducted by the British No. 2 Operational Research Section, after the Normandy campaign, concluded a Sherman would be set alight 82% of the time following an average of 1.89 penetrations of the tank's armor. United States Army research proved that the major reason for this was the stowage of main gun ammunition in the sponsons above the tracks. At first a partial remedy to ammunition fires in the M4 was found by welding 1-inch-thick armor plates to the sponson sides over the ammunition stowage bins. Later models moved ammunition stowage to the hull floor, with additional water jackets surrounding the main gun ammunition stowage. The practice, known as "wet stowage", reduced the chance of fire after a hit by a factor of four.[