Kakashi is strong, I agree. And after reading the thread opener, it does appear that Kakashi's chakra usage has become more refined. I believe many members here were saying that Kakashi had too much chakra, actually, during those fights. Also, let us not forget that Kakashi was a candidate to be the hokage, meaning that he himself is at the kage level.
With that said, if Kakashi were to fight each of the kage 1 on 1, he would have a hard time with each, and the reason for that is Kakashi lacks attacks with a large area of influence. Kakashi is primarily a close-medium range fighter, hence his assignment as the leader of that division. As such, it is hard for him to overwhelm an opponent. Kamui is often referred to as the best technique, but if we focus on how Kakashi has fought up until now, we know that he rarely uses the move in a fight, which is understandable because it drains his chakra unnecessarily. He would depend on his usual methods. If this was not the case, he would have warped the deva path instead of trying to other means of defeating it, and he would have warped enemies on the battlefield (reanimated ninja, reanimated jinchuuriki.) If we will are to pit anyone in Naruto against someone else, let us keep their fighting styles/ way of thinking in character, without adding our fourth dimensional, logical thinking.
Now, for the matchups.
Kakashi vs raikage: Raikage has overwhelming speed, so Kakashi would probably start off with using a clone and watching from some hidden spot to look for a weakness in the armor. He would see that physical attacks would be worthless, as well as his raikiri. After a short while A will defeat the clone through some form of high speed attack, and Kakashi's analysis should be complete just around that time. The only way to defeat A is to use Kamui. Kakashi will once again set a distraction, and have a clone attack A. A will assume that it is another clone and defeat it, but this is what Kakashi would have been waiting for. Kakashi would probably pick up, as we may have, that A pauses and leaves himself open the moment after he completes an attack. If Kakashi is able to lock the time-space barrier of Kamui onto A at that moment, then he has the chance for victory. However, if A notices the attack, and successfully dodges it, then he will not fall for it again, and chances of Kakashi's victory will become very low.
Kakashi vs mizukage: Kakashi would be more willing to engage Mei in close quarters combat, because he seems to be faster/more adept at taijutsu than her. In fact, Mei's preference for a ninjutsu battle would be key to her potential defeat. Mei would open the battle with her lava release, evidenced by..just about all her other fights. Despite the somewhat wide range of the attack, it is not something outside of Kakashi's ability to dodge. At some point Kakashi might let Mei believe that a lava attack hit him, perhaps by having a clone scream, perhaps with a replacement log. However this is where Kakashi's real counterattack would begin. After positioning himself close to Mei through earth release movements, Kakashi would emerge from beneath the ground and keep the fight close range. Mei would be pressed to defend herself, and unable to use ninjutsu. Perhaps they kunai clash and leap apart, giving Mei time to create seals for her water release: water dragon bullet (I doubt that she would use hidden mist against the man that defeated Zabuza in the mist.) Kakashi could counter with his own water dragon or an earth wall. This match would be decided without kamui, and most likely in Kakashi's favor. The Kakashi that Mei was engaging was a clone, or a clone was left underground to grab her ankles. Raikiri is charged. Raikiri is plunged into Mei's chest.
Kakashi vs kazekage: Gaara has defeated his weakness from the time that he fought against Sasuke's chidori, and that is his mobility. Gaara will begin the battle floating on his sand above Kakashi. From this position he is protected from Kakashi's usual methods of attack, as well as Kamui (he has better maneuverability than Deidara, and his sand automatically defends him.) Kakashi would most definitely not attack Gaara upfront, as he would be overwhelmed by the sand. He would probably attempt the same tactic as he would use against A, by using a distraction and observing the fighting from a distance for any weaknesses. This will most likely not work against Gaara, as Gaara is familiar with Kakashi's fighting style, and Gaara is a sensor through his sand/ third eye. Gaara will eventually find Kakashi, and being good at manipulating his sand in such a way that the enemy is left with little choice but to be caught, he will catch Kakashi. Kakashi's lack of wide range attacks (or lack of demonstation of such) will be his downfall. And if they are fighting to the death, Gaara will most likely crush Kakashi. Gaara is also very adept at using clones as feints. In fact, I argue that he may even be better at it than Kakashi. This match is most likely going to Gaara.
Kakashi vs tsuchikage: This fight will play similarly to the Gaara fight, as Onoki is mobile within all axis, and has the use of rock clones. The name of Kakashi's gameplan would have to be "do not get touched, and hit him with a counter.) Kakashi will need his clones for this. Onoki would probably attempt to jinton Kakashi, and not the square shaped one-man vaporizer, but the cone shaped destroyer of wide areas. But, Kakashi would be prepared for that and be hiding somewhere else, allowing his clone to take the attack. Unlike Gaara, Kakashi would have many openings to take advantage of. Onoki's back problems, Onoki's idle floating, and Onoki's need for contact to increase/decrease the weight of something. All of these opportunities could be used to kamui him. He showed that he would have been incapable of dodging even the water bullet from the nidaime mizukage. If Onoki stops moving, it would be over for him. At the same time, if he can locate Kakashi, jinton cone from the sky would end everything in a clean way.
Lastly, Kakashi vs hokage: Kakashi and Tsunade are familiar with each others abilities. Tsunade is strong and the premier medic ninja of the current shinobi world. I think I summed up her abilities quite nicely in that one sentence. As in all of her fights, Tsunade will attempt to take Kakashi out head on, as she has no long distance attacks. This would probably be the easiest of the fights for Kakashi. If he eats ANY attack, the fight is instantly over, but he has the sharingan to help with that. He also has an assortment of ninjutsu to keep her on her toes. Her resilience may be an issue, however. Lightning, fire, and water damage can all be healed, and Kakashi has displayed neither any effective earth release attacks, nor any wind attacks at all. Getting close to use raikiri would be a messy suicide, as Tsunade has shown the ability to fight despite devastating wounds (two susanoo swords to the mid section.) Kamui would have to be used to end this fight, and like Madara said, Tsunade is stronger than the raikage, but not anywhere as fast as him. Kakashi can probably manage to trap her in the barrier after halting her movements. The only question would be if he could halt her movements. Tsunade likes to pause after all of her attacks, so that would probably be the best time.
These are my opinions on how each fight would be carried out. Sorry for the length.
