I think either obito or Izuna or Izuna using Obito
Lol good guess but can't possibly be right about obito, since when tobi and minato fought obito had recently died. Doesn't fit the time line.
Maybe tobi is madara's younger brother, i remember that it was said that madara took his younger brothers eyes. Tobi has two different eyes so maybe he took the sharingan from obitos dead body and the rinengan from nagatos dead body.
That's one way of explaining why he has two different eyes, or why he was using madaras name.