What is so good about Good Friday?

Isaiah 52:13-15

13See, my servant shall prosper; he shall be exalted and lifted up, and shall be very high. 14Just as there were many who were astonished at him—so marred was his appearance, beyond human semblance, and his form beyond that of mortals— 15so he shall startle many nations; kings shall shut their mouths because of him; for that which had not been told them they shall see, and that which they had not heard they shall contemplate.