I was talking about tsukiyomi as a whole and not itachi's tsukiyomi.
But you pointed it out correctly, itachi's tsukiyomi can indeed defeat even a jinchuriki unless proved otherwise by kishi.
But its not going to end the way you are thinking...
If itachi manages to cast tsukiyomi on a perfect jinchuriki and makes it unconscious , still the biijju can take possession of the human body and defeat him.
It took Itachi alot of chakra and he had to run immediately, which means he was not in condition to fight after using MS.
Itachi says that the MS gives its user the ability to gain control over kyuubi.
And also perfect jinchuriki are immune to genjutsu.
Now i want to say why obito(assuming the fact that he controlled yagura) is best genjutsu user.
Obito managed to gain control over kurama in 3t sharingan. A feat no one has ever achieved . Not only that, he also fought minato, the 4th hokage and controlled kyubbi simultaneously which is a quite big achievement as, itachi was always immobile whenever he used his genjutsu, which means he never moved an inch while performing genjutsu or lets say tsukiyomi for convenience (since he genjutsu'd kabuto pretty good) .
Also itachi never was able to gain control over kurama, and even if he could, he could never be able to do it with 3t sharingan, but guess what? obito did it with 3t sharingan, so who is better now ?
Obito also controlled yagura, who was said to be a perfect jinchuriki.
This is also a remarkable feat, because he controlled two living beings. And that too for a long period of time. And not to forget he did it in 3t sharingan only.
Even a basic tsukiyomi couldn't do that and it also uses up alot of chakra.
so obito managed to some stuff with 3t, which is even difficult to do with MS.
That is why i think he is the best.
Also both of the points can be used for madara also.
As he fought hashirama while controlling kyuubi and minato the genius confused obito with madara.
