It's a toss up for me between Obito Sasuke and Orochimaru
Obito: The prophecy has been pointing to him for a long time to be the harbinger of destruction. The parallel between him and Naruto is just itching for him to be the final bought.
Orochimaru: Why else would he come back, he played his villain role far too long to just change his ways. I feel as though he has a ticket to become the final villain, as far as the prophecy goes, I'd say he'd have to control someone such as Sasuke or Obito to fulfill that part.
Sasuke: Naruto's rival and friend ever since the beginning of the show, those two have been butting heads and pulling each other along to get stronger. Sasuke has been tainted with evil and I believe he has yet to show his true intentions. He desires power and has a "plan" to fix the future by deleting the past. The tie between Naruto, Itachi, and Sasuke will be continued until the curse of hatred is over and the wishes of all of them have been fulfilled.
Kishi has been doing a great job of leading us to believe that multiple people could end up with the role. Who knows!