I say Itachi wins with high difficulty. Tsunade has shown evasiveness as she demonstrated during the Kabuto fight. She hasn't dodged as much against Madara because he has only used attacks that pierce the body and with her byakugo or whatever she is able to repair that damage and therefore doesn't need to dodge.
Itachi's biggest means to victory is in the form of Amaterasu. Tsunade has a good chance of dodging attacks; however, I anticipate she will eventually be hit. At this point all Tsunade can do is manage to kill Itachi before amaterasu burns her to death (this would be delayed because of the byakugo's regenerative properties). During the duration of Tsunade's burnout she has to get to Itachi and hit him.
This is hard to accopmplish because of Susanoo. Itachi's is unique in having the Yata mirror and she can't punch through as it block all attacks according to the manga. So Tsunade has to get behind the Susanoo without getting hit by the sealing sword of Tetsuka. This isn't entirely impossible, but highly unlikely due to Itachi's intelligence.
To those who say that Tsukoyomi is instant game over, Tsunade can summon Kastuyu, and have her separate and attach a little piece on her. Katsuyu then is able to surge chakra into her and stop genjutsu
So in the end it is can Tsunade hit Itachi before hit by amaterasu...the chances of her doing that are relatively slim.
