Characters Kakashi can beat? Here:
1. Obito (unless he has immortal jinchuuriki paths with Sharingan and Rinnegan and Gedo Mazo at his side)
2. Possibly Itachi
3. Anyone from Akatsuki apart from Pain
4. Zabuza
5. Tsunade
6. Possibly Jirayia
7. Other current kages
8. Base Naruto, possibly SM Naruto as well
9. Possibly Sasuke
10. Any Konoha ninja besides Naruto
11. Killer Bee
Hmm, I think these are the key players Kakashi could beat.
Now, who would rip Kakashi apart?
1. Nagato
2. Danzo (Kotoamatsukami)
3. Madara
4. Hashirama
5. Current Naruto
6. KM Minato (although it's arguable if he shoud be counted since he only has this power as edo)
Have I forgotten anyone?
As a rule of thumb, however, Kakashi can theoretically KO anybody with Kamui with only few exceptions (Nagato due to having more than one body, Danzo due to having a more powerful OP technique, Obito due to having a counter).
Yes, Kakashi WOULD also stand a chance against someone like Madara. It's generally not in character for him to use Kamui right off the bat but when he is certain of his opponent's overwhelming power he does not hesitate to use it at the very start of the fight.