My turn? K, cool.
Kakashi is one of the all time great characters. "Great" nothing more.
I love Kakashi more than most characters and would head his fan club if i had the energy to start one.
But let's face facts. His chakra is gobbled up when he uses the Sharingan, simply because his body wasn't made for it. In theory, he would be able to one shot anyone with Kamui, but who's really gonna get caught by Kamui with full awareness of the jutsu? Not many people. Kakashi is indeed a deadly opponent, no doubt. His name obviously carries weight, but he isn't too special. He is a strategist. Only to be outdone by the Nara's, (In terms of leaf ninja.) He has plenty of jutsu to choose from and the intelligence to time it. Making combos that are overwhelming, but he doesn't have to chakra to drag on a long fight. IMHO, if Kakashi had more chakra, he would be on par with Itachi, but he doesn't. You cant get much stronger than your chakra reserves will allow. It's near self destructive. Kakashi is a support character, it's what he does. If you have him on your side going into battle your chances of coming out alive are much higher.
He is "over-rated" because he is seen fighting more than alot of characters. Or rescuing those in need. He is insightful, sharp, and strong. So much that he is candidate for Hokage, which he became in Tsunade's absence.
As to whether or not he could beat the Kages... Maybe. Not definate at all though.
Correct me if i'm wrong. I love an open friendly discussion.