Madara is capable of being a final villain, but another viable option could be Orochimaru.
Orochimaru always was the best villain of Naruto in my book, but I'm concerned if he is still able to be a threat to the main characters with the current power levels. It all depends on his final plan, but since he is Orochimaru I believe he can come up with something.
Now, who I don't want to see as final villain. Obito and Sasuke, due to how predictable their final demises would be.
I don't want someone that everything is pointing out is going to be defeated with TnJ for final villain. This is a shounen anime, so it is predictable Naruto is going to change Sasuke's mind during their battle, the same for Obito since his backstory and motivation were explained.