I was in a thread where this the king of all morons stating that Kyuubi is not Madara's prime. There were also other people who think the same. I am here to simply state why Kyuubi is actually part of Madara's prime.
The definition of Prime is: A state or time of greatest strength, vigor, or success in a person's life.
@bold: Like in the definition, it is when someone in his greatest strength he could ever possibly be and that would refer to EMS Madara because with Kyuubi, he was actually stronger when he doesn't have it. The attacks Madara can combine with Kyuubi are stronger than his own attacks for example:
the slashes of PS alone are mountain slicer at best, however when we look at when EMS Madara combines them with Kyuubi TBB, this is the outcome:
they easily reached beyond mountain level. So by that, we can say EMS Madara attacks are wayy stronger when combining them with Kyuubi.
Now what makes Kyuubi part of Madara's prime?
its the fact that Madara had actually used his power (Sharingan) to take control of Kyuubi , the same thing Madara did to Kyuubi . So EMS Madara not only used his power (Sharingan), but he also used took control of it so Kyuubi became his tool to use. He also shown the ability to summon it as well as . Kyuubi is indeed EMS Madara's tool and to use.
Lets not forget, that Madara already said that Kyuubi is his tool:
We should also use Kabuto example when he said that his Edo Tensie shinobi are part of his power:
Kabuto summons and controls them. Same method Madara used on Kyuubi. So Kyuubi was indeed part of EMS Madara's prime.