Until the end of 1943, T-34 was the king. After Panther showed up(which was a heavy tank, not a medium one) T-34 regiments had difficult time taking on panthers.
Panther was a great tank, but it had gaseline engine, which had severe mechanical problems. Also, because of allied bombing, germans had trouble producing high quality armor(i dont remember the whole story). So Panther's armor wasn't that much good.
My choice: T-34.