I usually lean towards Vito,for 2 reasons
1 He started the whole empire.He came over with nothing and built a large and prosperous business(not getting ionto the methods he used....)and
2 He was able to keep his family together through it all. Something Michael was never able to do was keep his business out of his home life.His wife left him,and his children grew to resent,for lack of a better term,him.