I'll side with Tobirama. While I don't see how he gets past Susano'o, I don't see how Susano'o lands a hit on him.
- Genjutsu easily maneuvered with his speed and use of shadow clones. Itachi won't risk wasting chakra in such a fight while holding up Susano'o
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- Amaterasu? Someone who reacted the Obito in his Juubi form and surprised him with his speed? His sensory skills making it easy to detect the build up as well "I sensed"
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which enables him to even react faster since he'll be prepared. Matter of fact, this can also be used in his combat against Genujutsu.
Without Susano'o Itachi doesn't stand a chance of facing Tobirama. His reflexes and speed are on another level, Itachi cannot afford to come close or he gets tagged
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and even the likes of BSM Minato acknowledged this
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-- Itachi dies a bad death if he attempts such and add clones on the battlefield, his ability to teleport to them and even certain areas in the battlefield if tagged. With that being said, Tobirama easily outlasts Itachi with his Susano'o on and takes advantage once it's dropped. All I can think of right now.