Reanimated Itachi? or Itachi before he died? Because if it's the reanimated version Sesshomaru wins with Tensaiga alone, if it's Itachi before he died... He would put up a good fight, but over all seeing as how he's primarily a Genjutsu user and Sesshomaru is a bad ass Demon, the Genjutsu would ultimately be useless against him... and then Sesshomaru could easily take out Itachi with... lets say... the Tokijin, or just by Transforming into his Demon form and destroy Itachi. Either way, my vote goes to Sesshomaru!