At the height of his power I would fear Vito. He built a powerful family from nothing, and had the protection of the politicians and the police to back him up, as well as the feared Luca and a very competent Clemenza. It would make sense to accomodate Don Vito if possible, and to his credit Don Vito knew how to do business with as little public bloodshed as possible. This does not take anything away from his ferocity, however. Don Vito took his revenges out himself-killing Don Ciccio and his guards in Sicily, and others.

Michael was to be feared because of his sociopathic tendency that developed in the first two films. (I haven't been able to finish the third so my comments on him may be flawed) His ability to order the murder of anyone remotely in his way or to humiliate or destroy anyone who remotely offended him. But we also see Michael's self-destruction as a human being. Whether his enemies could see this I can't comment on for part 3, because he had killed or dismissed pretty much everyone in his way at the end of part 2.

Point being, Vito remained strong while Michael went into a downward spiral, so I give it to Don Vito in a close contest.