Well excluding Sage of six paths:
Top 10 list
1. Ten tails host Madara
2. Ten tails host Obito
3. SM Hashirama/ (including ET: Kabuto, Orochimaru)
4. Half Kurama BSM Naruto
5. EMS Sasuke
6. Nagato
7. DSM Kabuto / Minato
8. Itachi
9. Tobirama/ Killer bee
10. Third Raikage/ Muu
11. Kakashi/Gai
This is how I could see them
Killer Bee is a sitting duck if he goes full 8 tails, if he's smart he won't do that. In base, I can see Killer Bee being a problem though. 7 swords style can't be countered with Kakashi's kenjutsu slash taijutsu skills. But he's not as rash and stupid as sasuke, he won't just dive in and try and beat him. Itachi showed that the correct tactic against Killer Bee is to back off and avoid getting too close. Using Lightning beast running jutsu will force Bee back, or a Water Wall.
I've said it over and over but I think Kakashi surpasses him, skill wise they are the same. FTG can escape Kamui but Kakashi has intel on FTG. Tobirama doesn't use as many kunai as Minato, If Kakashi gets rid of his kunai, then he has no place to teleport too.
Third Raikage is Kakashi toughest opponent imo. Kamui is strong enough to rip his body parts off, and the sharingan can read his movesets since Sasuke was able to read Ay's movements in V1. No doubt the Third will charge at kakashi to try and stab him, Kakashi can summon an earth wall to block his vision. Kakashi can then kamui the spot of the earth wall he charges through which surprises the Raikage. WIthout kamui, Kakashi can't win.
Mu isn't difficult. The sharingan has proved to see the Rock villages invisibility techniques, 2 tomoe Obito saw that rock ninja and stabbed him. If Mu goes invisible, Kakashi's 3 tomoe sharingan should see him. If Mu tries to use Dust release while airborne, it just makes him an easy target. Kamui is much faster than dust release.
I put him just below Itachi.