i strongly believe its Shisui Uchiha for various reasons. First of all i have a theory, according to the manga you have to kill a person close to you in order to get the mangekyou sharingan. This means that shisui had to kill someone close to him and this would mean he is a murderer. If he had actually done this, he would've been captured by the police as they tried with itachi. according to my theory, you don't have to kill someone close, but to suppress all emotions. the suppression of emotions is then related to the assassination of a person close to you because to kill that person you have to forget about emotions. I believe Itachi and Shisui got their mangekyou this way and then as seen on the manga, shisui handed over his eye to itachi and lost the other one to danzo. Its been said that there are no traces remaining of his body which could mean he was never found dead in the river and was just assumed dead. Shuisui might have implanted two other sharingans in his eyes and after discovering what madara had planed, decided to take over madara's plan and started to act as if he were madara. Besides from this theory , there are some physical similarities, for example, madara had long hair and tobi had short hair Just like shisui. the body flicker technique could also explain some of madara's teleportation and speed, and shisui was known to be a master at it. but as madara does appear during the birth of naruto with long hair, i could say that he was still alive at that time and he then thought shuisui everything before he died, assuming he died somewhere between naruto's birth and shuisuis's apparent death