I think the best Don of all was Vito Corleone.
Only Vito could be the best, because he created and made the organisation as big as it was. Michael was certainly a man who deserved most of all to succed the Don, but we should not forget who teached him to apply his houge skills, he succed from his father.
Not everyone could rule the organisation, I think it was in their blood to be such perfect Dons.
it's hard to decide if vito or Michael was a better Don, they were both perfect in their generations.
Finally, I think its correct if i support the first Don and Creator of the Organisation and Michael.

In terms of Vincent, I neither understand why he became Don. I perceived him as inapplicable for this hounor as his father. He didn't have the same "Don skills" as Vito and Michael Corleone.
I think he just was the onlyone who wanted to be a Don and had a bit experience in criminality.
The best end for Gf part 3 would be, that michaels son the organisation continued, but instead of that he became a musician.