Interesting.
Michael is the main character. He is the one who evolves (Chirs Moltisanti's "Arch-speach" comes to mind).
BUT, Vito is the one who is referred to as The Godfather. The one who actually holds the title of Godfather (Moe Green referring to him as The Godfather).
Micheal is referred to as Godfather sometimes (by Lucchesi in part II for example), but never as The Godfather, at least not to my recollection.

For me the only Godfather in the movies is Vito. Michael whole "career" he tried to be like his father. He tried to protect his family, like his father had done.

Insomma : Michael tried to be as The real Godfather was.

Well that's just my opinion.

EDIT: DonMichaelCorleone: That was actually a great point! That it's the title rather than a person. Very interesting.