For those who CBB to read - Read the Bold
Now as we all know, realistically the true strength of a tailed beast and his Jinchuuriki is not determined by the actual strength of the tailed beast, it is a combination of both of their abilities and their choice to work together that allows them to become powerful.
This removes the factor of the current fight during the war - as all tailed beasts (At least up until Edo was Released) have a Jinchuuriki to work with.
Now that is clarified we can begin to talk about the chakra available to the tailed beasts and what they really are.
I recently read a thread by @Blaze Release Wherein he determines that the tailed beasts are merely forms of spiritual energy - and do not have a physical body.
After understanding this theory, it is important to acknowledge that generally, the power of each tailed beast BY THEMSELVES (With no Jinchuuriki) is determined by the amount of tails each beast has.
This is further supported by the 8 Tails comment about the 9 tails - that the power of a Tailed beast is not determined only by the amount of tails but by the ability to work with their host. This informs us that Kurama is not working with his host at the time - he is weaker than other tailed beasts that do.
Therefore Kurama's Arrogance causes him to be weak (Alone)
In addition, the most important factor to acknowledge about Kurama's power is that his power was cut in half.
During the battle with the 4th Hokage (Minato), the 4th uses the Reaper Death Seal to seal half of the 9 Tails Chakra - evidently causing him to become halved in size.
The other half of this chakra was then sealed within Naruto - and remained with him for the rest of his life.
Now, to my knowledge, the other tailed beasts did not have any of their chakra sealed or removed from existence through being sealed - only the 9 tails lost any of his original chakra - which was sealed - halving him in size.
Therefore the chakra of the 9 tails has been halved whilst the other beast's chakra has not.
The question is up to whether or not you personally believe that the chakra of the 9 tails would be recreated in existence - (similar to what would happen if the tailed beast died with its host.)
The difference between a death and a seal, is that when the chakra is sealed - it becomes unavailable to the world - just as sealed souls are no longer available to be used in an Edo Tensei.
Therefore it is plausible to substantiate an argument that states that only half of Kurama's actual chakra is sealed - and therefore - available to Naruto, making Kurama, by himself, NOT THE STRONGEST in chakra either.
To my understanding the list might look (based on chakra) a little like this.
1, 2, 3, 4, (9), 5, 6, 7, 8
----------- 4.5------------
What do you think, and do you believe that Kurama regains his chakra over time / his being is regenerated as his chakra 'died' in the RDS?
edit: Everyone is comparing Kurama's TBB to the other tailed beasts, but has anyone thought that the other beasts werent using their full power because Tobi couldn't control all of the beasts normally anyway? It would make sense to think that if he cant control them in full beast mode then he wouldn't allow them to use overpowered TBB that he cant fully control.
