I will join this argument (because it has moved beyond the realm of debating) and offer my opinion.
Kakashi is strong, with or without the mangekyou sharingan. His kamui ability is also very powerful, being a mixture of a space-time ninjutsu and a barrier ninjutsu. It has also shown that it's speed is remarkable, as many have pointed out. However, I do not think that Kakashi can get the title of the strongest character in this manga solely on that one jutsu and kyuubi chakra. So let's build a scenario to prove that.
You are claiming that Kakashi, through a combination of kyuubi chakra and his kamui, can instantly defeat any character. So in that case, let us pit him against Naruto. Kakashi is ready to fight, with his mangekyou sharingan out. Naruto is ready in his bijuu mode chakra cloak. Kakashi summons the kyuubi chakra from within himself... wait, that is something that only Naruto can give him. Kakashi, pitted against Naruto, has no means of accessing kyuubi's chakra. Naruto will move at speeds that Kakashi cannot even perceive and beat him to death. You may argue that Kakashi can teleport to the next dimension where he will be safe, but that means that he also cannot attack Naruto. Whenever he comes back, Naruto will be there waiting to beat him again. Plus, it would appear that teleporting himself to and from that next dimension is VERY taxing on Kakashi. That is Kakashi's weakness. Kakashi, even with the kyuubi chakra, was able to teleport himself back from the next dimension, teleport gyuuki (missing one of it's tails) and bring gyuuki back. In those three moves Kakashi had used up all the chakra that he could afford to put into Kamui.
As for the speed of kamui, it does not appear to get faster despite the kyuubi chakra. When he was teleporting the Naruto clone into the next dimension, Obito could clearly see the warping affect happening to the rasengan. It was not instant. Also when trying to warp the gedo mazo's head, if it was instant, he would have been able to, but Obito was once again able to see that he was trying to warp it away, and had time to stop his fight with Naruto long enough to halt the jutsu. With that said, I say that kamui is fast and very powerful, and perhaps in the hands of its original owner Obito it could have been a devastatingly over-powered move. But in the hand of Kakashi, it is not enough to make him the best.
If Kakashi's kamui is that powerful and fast, I look forward to when we are next shown the battlefield again where we are shown a defeated Obito, Madara, and ten tails. If that is NOT the case, however, then you have to accept that you are mistaken.