minatos wasnt THAT brave. his reasons were kind of cowardly. "balancing the bijuus"? "cant compare to kushina"?
id say hiruzens sacrifice was the bravest especially since it was totally in vain and oro survived.
tobirama is second bravest. he went into battle by himself as a decoy vs 20 elite kumo jounins knowing he was most likely going to die, to save his comrades.