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Anyway if its nagato vs itachi ill say itachi, simply because nagato hardly has any battle experience fighting as nagato and because of this hasnt really experienced the pressure of being in a life and death situation.
Also because of this he hasnt trained his mind and body.
If its 6 paths vs itachi, ill lean more to the 6 paths. Only thing im not sure of is that tsukuyomi takes effect though your eyes and we know that all 6 paths share vision, therefore its fair to say that all of them might be caught in tsukuyomi and if this happens they would then be vulnerable to the totsuka sword straight after tsukuyomi has ended