My vote goes to Madara as well. One of the Original founders of the Leaf, he grew up in times when war was all that was known. Viewed and an equal to the 1st Hokage. Everything we know about the power of the Uchiha clan we can trace back to this man.
I would say Madara Uchiha would be the most likely winner. Pain possesses only the Rinnegan, where as Madara possesses the Rinnegan and the Sharingan/Mangekyo Sharingan. Not to mention he is capable of Summoning the Nine-Tailed Fox, which was the tailed beast that Naruto tapped into to defeat Pain.
