This match would be an easy win for Itachi, and the reasons are..
Think about it a little bit, when A and Sasuke fought, Sasuke only used his Susanoo the first time.. it was not even yet completed it's first form.. it was just a bunch of small bones around him, no shield or bow, and basically nothing except the bones..
Reasons why Itachi would win:
When A was fighting Sasuke, A almost used all of his strongest physical attacks against him, and none of them actually harmed Sasuke that much, and the reason is "his" Susanoo..
Susanoo actually has 3 levels.. here are the examples of Itachi's levels of Susanoo..
The first physical look is the big completed form of bones:
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The second look is the face and some kind of energy covering the bones, and that adds much more defense.. (check the fight on the anime, you can see the stages much better)
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The third and the completed form, which has some kind of strong energy armor covering the whole Susanoo, and this last level of Susanoo is in no way to be broken through:
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Now let's see what Sasuke's Susanoo looked like when he was fighting the Raikage..
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It was not even yet completed it's first form! it was barely just a small bones surrounding him, and the Raikage couldn't even break through it with all his powerful attacks..
Here, you can see a comparison between Sasuke's Susanoo and Itachi's Susanoo in terms of levels.. you already saw Itachi's levels of Susanoo above, so here are Sasuke's ones..
First level, he achieved it at the end of the fight between him and the Raikage:
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The second look of his Susanoo is when he fought Danzo, you can see all the energy that covers the bones and the face of Susanoo is being define, also.. he got his weapon which is the bow:
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The third and the last look of Sasuke's Susanoo is when he got pretty pissed when he was talking to Kakashi, and here where he's Susanoo has achieved it's final stage, just like Itachi's one:
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You can see how it looked exactly like Itachi's Susanoo and has some kind of energy armor covering it, but sadly.. he went blind after using it lol.
Now think about it, if Itachi had to fight the Raikage, his Susanoo would be at it's final stage from the beginning, and not only that.. Itachi's Susanoo actually is far more powerful than Sasuke's Susanoo..
With all that defense Susanoo originally has, Itachi's Susanoo has (two) legendary weapons, the Yata Mirror and the Sword of Totsuka, which simply makes him (invincible..)
Yata Mirror: can repeal all attacks, and basically nothing can go through it, not even the tailed beasts bombs can scratch it.
Sword of Totsuka: the most powerful sword ever, when he stabs someone with it, he'll seal him forever, just like how he did with Orochimaru.
Now this is only a comparison between Sasuke's and Itachi's Susanoo, you all know how Itachi is far more skilled than Sasuke in terms of everything, Genjutsu/Ninjutsu/Taijutsu and basically everything else, I wouldn't even think that the match between A and Itachi would need to go as far as if Itachi would need to use his Susanoo, really.. he's much more powerful.