I hope you do realize that in ANY fictional 1 on 1, unless you're specifying the handicaps and advantages, both fighters start neutral. To put it simple for you, Kabuto starts facing his opponent and has
NO EDO TENSEI SUMMONED YET.
He's vulnerable to genjutsu and some fast attackers. Nuff said. I won't go over this against, because I'm currently debating this on 2 other threads and frankly if you won't bother to read them, I won't bother to prove anything to you. If you are open to the idea, here are the threads. I'm sure that if you look at it from a different perspective, you will find that there are shinobi that stand a chance to kill him. And it's Naruto, not DBZ. Characters have weaknesses even if they are OP.
threads:
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As I said, if you're willing to accept how a fictional battle works, that means Kabuto CAN be beaten. If you're biased, you will think kabuto + all his edo, while the opponent is tied up and cannot use more than 10% of his chakra will be a normal matchup.
Hello there Anub, nice to see you again.
Also, for everyone who looks at the first thread he posted, look at my arguments too, I believe they are compelling enough to prove my side.
Anub, you're arguing that Kabuto is not unbeatable. That is true. He can be beaten. But this thread is saying that he is the strongest ninja overall. Which is true. I have proven that much.
In any one v one battle, he will have the greatest chance of victory. Fast attacks will most likely very rarely work, because there are no virtually instantaneous attacks that can kill him if he can heal so quickly. The most that can happen is that a powerful punch or cut would be laid on him, and he can easily survive that.
In terms of genjutsu, he is vulnerable, but since he constantly takes in nature chakra, weaker genjutsu would not last long enough to provide time to defeat him (as seen in the last manga, where Itachi successfully used a weaker genjutsu on him, but he didn't get bodily harmed).
Only strong genjutsu can achieve victory, and the only two that COULD work are Tsyiukiomi and Shisui's genjutsu. All he has to do is avoid Itachi's eyes, and he avoids Tsyiukiomi. Shisui's genjutsu can be avoided in the same way. The only way Shisui's could work is if it were used on an edo, and then the edo surprise attacks Itachi. If that doesn't work, which as I stated in the "fast attack" paragraph above is very likely, he could easily freeze everyone with his flash, and free the controlled edo, easy peasy.
So basically, since those are pretty much the only scenarios in which he could really lose, and he has a great chance of overcoming them, he can confidently be proposed as the strongest ninja (when at his peak). The only one who could possibly be better (key word: possibly) is Tobi, since he has Gedo Mazo which is great for sealing, and also has his dimensional jutsu, which is also good for sealing. This could feasibly eliminate the edos, but that would still be immensely difficult.