It was never stated as being the strongest, however, a great deal of the more powerful characters in the series have taken some of his DNA in order to obtain his wood jutsu (Danzou, Tobi, Madara, etc) it has the ability to suppress bijuu and it was strong enough to block a point blank tailed beast ball from the kyuubi (the strongest bijuu) so it is pretty self-explanatory that other than yin-yang release creation of all things jutsu that wood jutsu is the next strongest. This is absoultely incorrect, no offense. Bijuu mode is the jinchuuriki borrowing the tailed beasts power and transforming into said beast. The jinchuuriki's power does not have any effect on how powerful the bijuu will become because they are simply releasing the bijuu's power and controlling it. I will agree with you that the intelligence of the jinchuuriki controlling the bijuu is important, however where is your evidence that they gain new jutsu? They don't. You say that the 9 tails (at full power) would not be able to take on the other 5 like bijuu mode is right now. That means that you are stating that naruto's power is greater far greater than that of the nine-tails yin chakra which was sealed in the reaper. Naruto is nowhere near that strong. Evidence he is not that strong, Naruto << 3rd Raikage = Eight Tails < Nine Tails, keep in mind that even Kyuubi mode Naruto was << 3rd Raikage, Naruto without kyuubi mode doesn't even phase him.