Casillas was past his peak by the age of 31, for a goalkeeper that is incredibly early. He was always a great shotstopper, but Gigi had a far better command of the box and his defence than Casillas ever had.
Buffon is simply the greatest goalkeeper since Yashin, yes I'm being slightly biased but the facts back me up, nearly 20 years of world class consistency is unparalleled in the sport. He had a bit of a rough patch from 08-10 but was still better than most.