i guess its the uchiha founder...if u read a lot about ninjutsu history and the SSS he had 2 sons, one who had his "eyes" and one who had stamina and long life the one with the eye later became the founder of the uchiha and his offspring gained a similar eye known as the sharingan later the brothers were asked about peace and how to bring peace by their father and each answered this way Uchiha: i would bring peace by power and ruling with strength
Senju: I would bring peace by love and harmony and understanding. so the SSS chose the Senju founder as his inheritor. since both are sons of SSS and they can live long and they know a lot of ninjutsu. since tobi also wants to bring peace but as we see by power and ruling with strength so to me its kinds of obvious