By greatest, I assumed that to mean who was the enemy that was most likely to overthrow the Corleone Family. With this in mind, I think it comes down to Roth and Barazini. Of course they were a threat in different times. I dont think that Roth was that much of a threat when Vito was in power. Since Vito gave Roth his start, and they worked together, I think he still had a bit of loyalty in him. When Mike came along, and the way Mike was handling things, I think Roth went against him.

I voted for Barazini. I think overall Barazini had the edge over Vito, mainly because he wanted to get in the drug business, and had the support of the other families. Vito knew these, and was smart enough to rethink his position concerning drugs. He was able to make concessions and still save face. I really don't think Roth would have been that much of threat, if Mike wasn't so self destructive. Yes he was able to squeak by Roths attacks on him, but only a suicide assasination was the way he got him. I don't think Mike could outthink Roth.