The sherman is not a good choice to take on the other tanks chosen in this thread, unless of course we are talking about the 17pdr/76mm Firefly. The shermans standard 75mm although capable of knocking out both of the other tanks was not generally used to take on the heavier tanks like the Panther, that was the Firefly's job. The sherman however was probably the most changed varient of any tanks of the war, being used as DD's, minesweeps, dozers, flammerthrowers, as well as all the funnies used on D'day.
While the panther was designed to counter the T-34 both the 76 and the 85. Good cross country capability due to the wide tracks and better in the mechanics side of the things compared to the other german heavy tanks such as the Tiger or the Elephant.
But very capable of stopping any Russian or American tank.
But she was far from invincible and here frontal armour didn't matter if she came up against an IS heavy tanks. This RUssian tank had the capability to blow the turrent right of the top of the panther, even if the shell didn't penetrate the armour.
The T-34 was probably the best all round tank of the war, especially in her role as a medium tank. She was fast, well defended with the sloped armour and very reliable just like the sherman. Many could be made and if I am not mistaken that number was around an estimated 64,000 at wars end through all variants.
You cant compare the Panther to the sherman or the T-34, since it was a heavy tank, it is like comparing a Tiger to a t-34, they are wrong weights, there fore different armour, main guns, crew, and naturally differnet roles.
But if I had to chose between one, it would be the panther.