Hi DFTC,
I know that Vito is actually "The Godfather" in the sense of the film, and in even more years to come will be thought of when GF comes to mind.
Yet, trying to set aside the fact that Pacino is my favorite actor, I still feel Pacino's character, not to mention his flawless portrayal of Michael, was far more impressive, showing a young sweet naive' man transform into a cold blooded/cold hearted murderer, made him the "best" move character.

Not to take away from Brando, who too was excellent. Just my opinion.

Like I say though, it is a tough choice.
TIS