Never said his was the strongest, only that Yata and Totsuka made him near invincible but that wouldn't protect him from earth base jutsus that can attack from under Susanoo, a Meteor from crushing him, Kamui from bypassing his defenses or COAT which would make the Yata, totsuka and Susanoo into nothing. Plus Itachi lacks the stamina to keep up a long fight against strong/smarts ninja.
Also so many people seem to forget Itachi had an illness which he had to take medicine to just stay alive and going all out would kill him as it did against Sasuke. Itachi have been going blind since 13 when he first awakened his MS so his eye sight is bad. Every cell in Sasuke's body ached him from using Susanoo and he even wondered how bad Itachi must have felt (since his illness made it worst)
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.Prime Itachi doesn't exist, Itachi didn't have the illness at 13 from what we know but Obito knew about it so it had to be from after he left to before part 1 time started. A ninja usually reach their prime around 30-45 years old.
