To be honest, it is just about impossible to make an unbiased 'strongest character' list. Naturally, you're going to favor some characters more than others, and it will at least subconsciously affect the order. So, you're all really just arguing opinion here, and that is completely useless.
Before I post my list, I want it known that this would be MY incredibly biased opinion. Please, no flames.
[Not counting the Sage, or his sons. Those are clearly a given]
1.) Hashirama Senju (Like the Sage, his powers are legendary. In the new Naruto game, it shows his fight with Madara, and the guy is a TOTAL beast!)
2.) Madara Uchiha (Again, legendary power. Just the mention of his name is enough to incite war. I do mean Edo-Madara, by the way; with Hashirama's cells, Rinnegan, etc.)
3.) Itachi Uchiha (If not in straight power level, than in intelligence. The guy's got a brain like a computer, and it gives him a HUGE advantage. Then, of course, he was MS, Izanami, Susano'o, Amaterasu, etc.)
4.) Tobi (I know that not much has been shown from him, but his confidence level has to be backed by enormous skill. Rinnegan, Sharingan, his 6 Paths Jinchuriki, the Demonic Stature, and his S/T Ninjutsu.)
5.) Kabuto (HUGE leap in power. (supposedly) perfected the Edo Tensei, dragon sage, Orochimaru's powers, etc.)
6.) Minato Namikaze (Again, the guy is a legend. Noted to be the fasted ninja who ever lived, was well versed in fuin-jutsu, as well as the rest of the skills, his S/T ninjutsu was a beast, and the guy created the Rasengan.)
7.) Nagato (Especially Edo-Nagato. Mastered all the Rinnegan abilities, and destroyed Konoha single-handedly (well, with his 6 paths, but you get it.))
8.) Naruto Uzumaki (Main character, obviously. Mastered sage mode, and finally mastered Kurama. I am sure that, by the end of the series, he will be the king. And I really don't like this character!)
9.) Sasuke Uchiha (Eternal MS, and the guy just leaps in strength after EVERY fight. I agree that he owes a lot to the people who have manipulated him, but that doesn't make him any less of a beast. I am sure he will be 2nd only to Naruto.)
10.) Killer B (Yes, I am putting him before A. Didn't A himself claim that B had surpassed him? Also a jinchuriki, and has YEARS of practice after gaining control. Amazing kenjutsu, and Samehada just makes him a bad-ass.)
11.) Jiraiya (I am sure that my love of this character affects my assessment, but I'm okay with that. He was also legendary in his own right, and was the teacher to a lot of the characters in these lists. His sage mode wasn't perfected, but he did use it will amazing skill. Considered for Hokage, so he is clearly Kage-level.)
12.) A (Fasted shinobi currently alive, as well as Hulk-like strength. That much spell with that much strength is a DEADLY combo. Strong enough to crack Susano'o's rib cage, and fast enough to get out of the way of Amaterasu.)
13.) Orochimaru (Another Sannin that was damn near immortal. Had a VAST knowledge of jutsu, and performed them with precision, and skill. I believe that his arrogance was his downfall, but it is hard to deny the fact that he was a power-house.)
14.) Tobiramu Senju (We really don't know too much about him, but the mere fact that he was related to Hashirama, and also became Hokage is proof enough that he was incredibly powerful. Was able to use S-class water jutsu with 1 hand seal. So he gets my nod)
15.) Hiruzen Sarutobi (I know that we don't actually KNOW anything about his prime, but judging from his performance in his old age, I would say he was powerful. Mastered over 1,000 jutsu, and had expert-level knowledge in all forms of combat. Excellent strategist, and leader. He MUST have been a complete monster in his prime.)
Again, this is my BIASED list. I am not claiming to know every single thing about every single character, and every single interaction between every single jutsu. This is just based on what I have personally seen, and my own opinions. Take it as you will.