We should know in just a few more months.
My guess? Sasuke.
The thing is, Naruto's superpower isn't the Kyuubi, it's his ability to stare everyone down and make them not only face the mistakes they've made, but to seek out a brighter future and open their hearts. Naruto is one of the best "shonen heroes" – those fighters who constantly seem to turn this week's opponent into next week's ally. Naruto has, thus far, built up a LOT of friends.
The way the plot is moving now, with Kabuto and Madara occasionally making appearances to throw off the reader, but Sasuke has been gone a very, very long time now… Well, let's just say that I think we'll get to see both a definitive Naruto/Sasuke power-level comparison, as well as a final team-up/redemption that allows sasuke to protect/counter Madara's abilities enough to allow Naruto to bat clean up and win the day. That, or the reverse – Naruto will sacrifice himself to save Sasuke's soul, and ultimately everyone else as well.
Who would win in a showdown between Naruto and Uchiha Madara?