After his training, Naruto probably. Though I don't think he'll just walk in, slap Pain and Pain dies, it'll be a long drawn out fight, where Naruto unleashes something nobody expects to defeat Pain. (Kishi would only let Sasuke come in and slap Pain and the world dies...heh)
I think Madara is weak right now, and doesnt have the power he used to. Maybe "absorbing" the tailed beasts makes him super strong, we can only wait to see.
I don't see Sasuke passing Naruto again, maybe get on par with him, and have a final battle at VoE, this time with Naruto on top, and Sasuke asking for him to kill him, but Naruto won't.
Kakashi still has something up his sleeve. Don't forget Kabuto/Orochimaru. Orochimaru's power and jutsu to go along with Kabutos strength, genius, healing power ect....that will be the final battle imo in the Naruto series.
Also, would'nt it be a twist, if Sasuke helps Konoha and brings Orochimaru back to help.....think about it....Orochimaru is trapped in a dimension that was stated in the manga to be true bliss....it's possible it would take the "evil" out of him and restore him back to the hero he used to be before he went power crazy.....