Itachi is still number 1, with Sasuke ready to surpass him, and Madara couldn't even match Hashirama without more power, but I can't really say anything until the story concludes... so for now (for me) it's Itachi, Sasuke and Madara in that order.
Sasuke will obviously surpass them all in the end... if he lives.
Without Madara, the current plot of Naruto would be completely different
There would be no akatsuki
There would be no MS Itachi
Sasuke would be a good boy
Obito would be dead
No ten tails revival
Naruto would not be the main character or he would at least probably not be a jinchuriki
Etc, etc
So my answer is Madara
Sasuke of course, in waiting for his PS
Why not?
That's like saying if there was no Christopher Columbus there would be no America, of course there would be America it would have just been found later.
Well, I look at it this way, if they are all in their prime then Madara would still be below them because each generation is supposed to become stronger than the last. Sasuke in his prime now might have actually been equal to or a little below Hashirama in his prime. We'll never know however, unless he suddenly attacks him.