The Godfather is a classic but it only would have been a really good movie without Brando.
Yes, the acting among the others are spectacular. the story is just wonderful. And the direction-c'mon its Francis Ford Coppala-need I say anymore?
But Brando really tied it all together. Who else could play Vito Corleone? Who? I can't think of anyone who would have come near Bando's greatness. It would have better without Vito if Brando was not going to play it.
However, it still could be a really gould movie without him-just not a classic.