Maybe a stupid question, but who do you think the title of the movie/book refers to?

Vito?
Michael?
Both?

What about GF2 and GF3 where Vito is absent? - is it just a title for whoever is in charge at the moment...or was it meant to refer to Vito and they just didnt want to change the title for the sequels.....

I think I would have to go with Vito. Although the title is obviously Vito's as the film begins, it ends with Michael also being called Don Corleone. The difference is that Vito was called Godfather out of respect, as well as affection. Michael never quite generates that simple and honest loyalty and love that his father received.

The difference between Michael and Vito is obvious in The Godfather and GF2 opening parties. We find Vito granting favors to the people from his old neighborhood, like Nazorine and Bonasera. Who is Michael meeting with? Senator Geary, and Frank Pentangeli from the old days is left waiting in the lobby. He obviously never earned the affection that went with the title of Godfather.

To quote Michael in the Senate hearing, "It was not an alias. 'Godfather' was a term of affection, used by his friends, one of respect."

I would have to disagree. I have always thought the film was about Michael and his rise and downfall to being the Godfather of the family. Even though in the first and second film, it gave Vito some background, I think it always emphasized on Michael and how different he was from his father and how he has claimed the title and unable to truly have the love and affection that Vito has received.

That is what I think -- many might share different beliefs as I do because Vito Corleone was such a memorable character, but I still think the film really centers around Michael.

Michael became the Don but never became The Godfather and I believe that's what the title may subtly imply.

I have always felt the movies were about the TITLE fo the Godfather and how each family member used that power and position

I think it refers to Vito.

Without Vito there would have been no empire for Michael to take over.

Johnny refers to Michael as the GF when he sings at the party at Michael's house.

But Johnny also calls Vito Godfather at Connie's wedding when he meets with him about the Woltz movie and also refers to Vito as Godfather at the Las Vegas meeting with Moe Green. 'Sure Mike, I'll do anything for my Godfather' or something like that.

Vito was the godfather, and in GF he's referred to several times as "Godfather," always with respect.
Michael obviously becomes the Don, and is bigger in many ways than Vito. But he's almost never referred to as "Godfather" in any way similar to the respect given Vito. Clemenza formally calls him "Don Corleone" at the very end of GF (and kisses his hand), but not "Godfather." As far as I can recall, he's called "Godfather" only twice: Once by Johnny Fontaine as part of his entertainment rap while singing at Michael's party in III, and contemptuously by Lucchese.
I think this is intentional. Vito was portrayed as a "traditional" Don: a criminal boss, but one who took his duties personally and acted like a godfather (even if it was to garner obligations and debts to be repaid). Michael was strictly a businessman (and a cold-hearted one, at that).

For what it's worth, Godfather 2 opens with the message:

"The Godfather was born Vito Andolini in the town of Corleone in Sicily."