Clean sheet records are not the stat for best GKs, it's the stat for best defense.
Valdez won Zamora trophy more than anyone in the last 6-7 years, you know why? Because Barca rarely let goals in as they always had possession. GKs can win **** all trophies like Golden glove, Zamora trophy if defense in front of you is disjointed or whole team is not good enough defensively. At least in Barca's case they rarely gave away ball which meant Valdez won the trophy more than any. He won it 5 times whereas Casillas when he was still the best GK won it only once.
Courtois was not the main reason for their run in to finals and League title, it was outfield players 150% work rate in all the matches and pure brilliance from some of their midfield players.
Courtois also made few mistakes but that rarely gets blown out of proportion in media as it happened in Spain and in CL low profile games.
He is up there with the best though but I wouldn't say he is the best or I wouldn't swap his for De Gea. Some of the saves De Gea makes, I don't see any GK improvising to make those saves.