I would say either high Kage or a level that should be between that and Hashirama's level.
He did, to his credit and in old age, take on Orochimaru and two Kage. It looks as though they were toying with him with a not-yet-perfected (in my eyes) Edo Tensei, but yet and still he did a better job than most.
I'd say he can stalemate two Kage, maybe three. Meaning that all three are killed in the struggle. I don't know the limits of his prime, and I find that it's useless to act like you have some psychic power to know his prime, and to bring his prime up as an argument because it was just a few statements made by him being physically weaker, his chakra being less, and having less stamina. The depths of them are still unknown, so who really cares? He's not Hashirama who founded Konoha along with Madara. He may be slightly greater, slightly lower, or equal to Minato, seeing as Minato one-shots even powerful Shinobi like Tobi and Bee.
So I'd really put him ANYWHERE between Tobirama and Hashirama.