Marlon Brando Al Pacino Robert De Niro I go with Marlon Brando.
If you are talking about the first movie I'd say Brando since he's on the poster.
I should have omitted the quotations, because I mean the godfather in general (but in those three movies)
In that case Al Pacino.
He easily has the most on-screen time compared to the others
The title character
Pacino