The name Itachi means death. Crows are bad omens of death. The author of the manga has made the character Itachi symbolic of death and gave said character death's qualities. Death cannot be defeated, overpowered, outsmarted, or controlled. Death doesn't seek violence. Itachi has all of these qualities. In the manga Itachi will never lose because Kishi wrote him that way.
Which is why ranking ninja is pointless. Tsunade, Jiriaya, and Orochimaru are symbolic of the snail, toad and snake. Snail, Toad, Snake is the japanese version of rock, paper scissors.
Tsunade> Orochimaru
Orochimaru> Jiriaya
Jiriaya > Tsunade
Sure Jiriaya is thought to be the strongest but he cannot beat Orochimaru so why rank them?