Zabuza has the upper-hand when facing Itachi if they battled right now. Why you might ask? Well, Itachi said the Hidden Mist would not fool him, but that goes against the manga, and the anime.
1.) Itachi has no counter for the Hidden Mist Technique. That means he cannot use genjutsu on his opponent because he cannot see his opponent. Kakashi only lived against Zabuza because Kakashi's ninken could sniff him out. They are sensory summons to an extent.
2.) Itachi has no sensory skills that I know of. He has no summons that have sensory skills that I know of. Counter that. He wouldn't be able to lay a finger on Zabuza.
3.) You Itachi fanboys always say, "Well, (Insert any name) has no counter for his Susano'o, overpowered shield, and overpowered sword. Well guess what? Itachi never starts a battle out in Susano'o. This is not edo Itachi we are talking about either, so he does have limited chakra.
4.) He has no counter to the Silent Killing Technique. That hole in Zabuza's sword, is ment for de-capitating. Itachi wouldn't be able to just magicly summon Susano'o in the alloted amount of time it would take Zabuza to kill him.
Please someone counter how Itachi see's through the mist, how he can counter Zabuza's sword, and if he can even see through the mist? Do not say he will just go in Susano'o, because the manga is proof he never goes Susano'o in the begining of a fight.