Minato.
It fits Minato's fighting style the best. He is a close-range fighter that relies on quick movements to counter his opponents. His intelligence and analytical abilities alone make him a formidable fighter, but he would be that much better with the Sharingan. Afterall, his primary offensive technique (the Rasengan) is little more than a super-powered palm strike, and like Kakashi's Chidori, relies on being able to read an opponent's movements and attacks.
Hashirama, Tobirama, and Tsunade would be able unlock the Rinnegan because of their lineage. Hashirama and Tobirama would probably be better at utilizing the Sharingan, but it would fit Tsunade's fighting style better. The other two are not necessarily close-range fighters and therefore wouldn't really need the Sharingan as much as Tsunade would in combat. In other words, the Sharingan would likely be wasted on Tobirama and Hashirama just because I don't think they would use it very much. They would have probably treated it like Kakashi did in the first part of the series. Tsunade on the other hand would be better at countering opponents and landing deadly strikes.
Hiruzen would probably benefit the least from having the Sharingan. He already knows thousands of techniques, so he would not need to copy them like Kakashi. He is also the most balanced Hokage in terms of combat skills. He has extremely powerful offensive and defensive abilities, and can engage an enemy at almost any range. Like Hashirama and Tobirama, I don't really seeing Hiruzen using a Sharingan very much if he had one. It would be a waste of chakra for him.
