Every time someone makes an Itachi post both fanboys and haters come crashing in like termites. Itachi is not unbeatable, he's very strong but still beatable. Saying a kage level can not beat him is pure fan-based theory not authors-based. Whenever someone mention Itachi's power, fanboys always go straight to his most powerful form and say it will work on everybody, and boom end of the story. They never mentioned how the others can counter it, nullify it whatever. They never mentioned how the enemy have something that even Itachi can't counter even with Susanoo. It's always yep Itachi will win, will win, will win. It only shadows your judgment of shinobi skills. And as for haters stop hating, the guy is extremely strong that's why others believe he's unbeatable. It doesn't make fanboy until they mention he's the strongest and no one, no kages, no famous shinobi can beat him.
For people who says that dust release won't work, think again. If it can hollow a whole island and yet it wouldn't work. Kisame's Samehada can just suck up susanoo's, because it's made of out pure chakra.
I am going to refer you to the databooks, where it describes the Yata mirror. The Yata mirror contains any elements possible and all elemental combinations. No attack containing any elements would be useful against the Yata mirror. Kirin -fails, Dust release - fails, raiton armor - fails, rasenshuriken - fails.
Also we are not sure if Susanoo is made up of chakra. That doesn't seem right as chakra isn't that great of a shield. I think it is a spiritual being, much like the Death God that comes from using the death reaper seal.
Itachi is beatable yes but only by a very limited number of people. Without his legendary weapons he would be even more beatable, but he has them and that nullifies so many people's abilities.
Here is a list of what you need to beat Itachi, without all of these things you will be hard pressed to stand a chance against Itachi.
Extreme speed or
an absolute defense - To deal with Itachi's insane speed, and to dodge Amaterasu and the Totsuka sword. Either that or shinra tensei, or the ability to absorb chakra, or the ability to regenerate (Oro and Edos only basically), or ST.
Large chakra base - to outlast Itachi if he chooses to hide inside Susanoo, or if he decides to spam MS abilities.
Large scale non-elemental attacks - so far only Nagato and Rinnegan Madara have this sort of firepower. Again the reason why is Susanoo + the Yata mirror.
An almost sure defense against genjutsu(regular genjutsu) - Itachi starts out every battle with genjutsu, he has done so and is a genjutsu specialist, you can't discount this considering he has beaten Deidara with it, and Orochimaru, and has trapped Sasuke, and Naruto in it before, and has gotten Kurenai in her own. This is a big must. So far only people with dojutsu, Samehada or are a perfect jinchurriki have a defense against this.
The ability to never make eye contact and/or long range capabilities - Itachi's Tsukuyomi is unbreakable unless you are Sasuke or perhaps Madara. If you make eye contact you are screwed. Samehada and being a perfect jinchurriki would not help because it is instant, and does not take place in real time.
Intelligence - It doesn't matter how powerful you are if you don't fight smart against Itachi he at least has a chance of killing you, his entire arsenal is made up of overpowered one shot kills. That means that if you leave an opening against him then you are dead.
So once someone has basically all of those then they have a better chance of beating Itachi then he does of beating them. There are many characters who have some or even most of these things, but sometimes if you are missing even one that is all the opening Itachi needs.
