Muu. All he has to do is fly around invisibly, and wait for Itachi to lose all his chakra, due to the fact that Itachi will have put up Susanoo the minute he understands he has no clue where Muu is. Muu can just fly home, put on the kettle, while Itachi is in the middle of the forest, protecting himself from Muu, who he believes is waiting to release a Jinton.
Izanami wont work, as it requires physical contact, genjutsu wont work, as he has to look in his eyes (or use finger). Itachi's greatest strength is his genjutsu, which is COMPLETELY NULLIFIED by Muu's invisibility, and ability to erase his presence. Only SM Naruto has been able to find him, due to Muu's "malice." Its a joke if anyone actually believes Itachi can get Muu in a Tsukyoumi or Izanami.
Also, why does everyone assume a Jinton is needed to kill Itachi? A simple kunai to the throat will suffice (if Itachi doesnt put up Susanoo). If Susanoo is activated, Itachi's chakra drains at a MUCH faster rate. Jinton reveals one's position, therefore it could be a disadvantage if your opponent somehow manages to survive Jinton.
Muu is pretty much unstoppable. Im surprised he isnt higher up on people's all time lists. In effect, his invisibility is as good as Tobi's intangibility (the thing which sets Tobi apart from the rest of the Narutoverse.) Only Sage Mode has detected Muu.
In short...
Muu's abilities are BROKEN, and wipes the floor with nearly EVERYONE in the Narutoverse.