Madara Uchiha makes the most sense, because who else whould be stronger than Itachi and Nagato? It can't be the previous Kages and he said he had no luck finding Shisui's and Jiraiya's DNA. Izuna shouldn't have his eyes, because he lost them before he died. and even if would have the eyes, I don't think MS Izuna is stronger than Itachi and Nagato.