Clearly the most powerful boss at the moment until Sasuke gets his power up, with Senju DNA, the Eternal Mangekyou Sharingan and finally the Rinnegan. This guy is so over powered its unbelievable and now with him riding the Ten tails like a donkey no one can get close enough to seal him, let alone get him into a position to be sealed. Also to get his final power boost of being the 10 tails Jinjuriki he needs Obito to sacrifice him self to give him a real body which seems unlikely. However as he seems the most powerful he has only just come into the Naruto series so he hasn't had enough presence to become the final boss.
Conclusion: Either Naruto or Sasuke will beat him or Obito will betray him or Orochimaru will be able to revive the Hokages and Harishima will kick crap out of him.
Well first of all he's an Uchiha which instantly gives him an edge on all other non Sharingan users, as well a newly aquired Rinnegan which he has not yet used apart from his Jinjuriki 6 paths. But my main reason why he is my favorite choice for final boss is he obviously has the anger and determination to do anythin to get the Tsukuyomi world so he can get Rin back, which i think means screwing over Madara and becoming the ten tails Jinjuriki himself.
Conclusion: Most likely candidate but his demise may come from Kakashi or Might Guy or Madara feeling betrayed, or Rin will bitch slap him in the dream world and be all like "now i no u did not go killin people because of me".
Ten Tailed Beast
At the Current moment the Bjubi is not fully under Madara and Obito's control and with power its wielding could easily over power Obito and give Madara a run for his money even in its incomplete state, if this happens anything could happen. at this moment in time i am unsure weather it will be the final boss as it is not fully complete (missing the Ninetails [kuruma] and Eighttails [Gyuki] chakra).
Conclusion: Will either Madara or Obito will become its Jinjuriki and Naruto or Sasuke or both will defeat them, or will run rampage and destroy the world.
Sasuke Uchiha
Although the awaited final battle between Sasuke and Naruto has been anticipated and will be the most exciting one, i feel that it may not be the final battle, as Naruto with his amazing power to change peoples hearts will change Sasuke's decision to destroy the leaf. Well lets face it Sasuke does tick all the boxes for final boss;
Epic justsu's [check]
Eternal Mangekyou Sharingan Rape eye [check]
Uchiha [check]
Naruto's boyfriend [check]
Theres only one thing for me that could up Sasuke's chances of taking on naruto and stepping up as lead boss man, would be if this place that Orochimaru is taking him gave him a large power up.
Conclusion: He will join Naruto and take on one of the other boss candidates, be the final boss and die with Naruto or get killed while fightin with Naruto against the final boss, or my favorite one big sex session with Naruto.
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