I have been wondering about this, but never took the trouble discussing this topic.
I will give stats about Shinobi whos name was mentioned the most.
Madara, Itachi, Minato, Obito even Sasuke is a Solo King nowadays.
I'm going to list each one of them to see exactly what they solod.
By solo I mean K.O, unabling enemy to fight for a certain amount of time or forcing the enemy to flee.
To keep the list short, im not going to name any fodders.
Itachi: Kakashi
Cant name any other, because Obito was mainly there during the Uchiha massacre, not to mention most of them were fodders.
Madara: Mu, Raikage, Tsunade, Gaara, Onoki, Mei, Gai, Hashirama, Kurama, Hachibi, Nanabi, Rokubi, Gobi, Yonbi, Sanbi, Nibi, Ichibi.
Obito: Konan, Kakashi
Minato: Obito, Kurama
Sasuke: Orochimaru, Deidara
Naruto: Sadly, I cant think of anyone without anyone inferring.
Madara is the clear Solo King as you can see. I hope all Itachi fanboys realise this, because Madara's list is only getting bigger.
This is how I did mine, there can only be 1 solo king alive so who was the first we seen (Never seen Sage or his sons)?
-Hashirama: was the first solo king, he beat Madara+Kurama even without using Bijuu 2-8 which he had at the time.
-Prime Hiruzen: only add him by hype, said to be the strongest Hokage and in his old age, Kabuto said he was the strongest Kage in chapter 140, pg 3 I believe, and that puts him over A, Oonoki, Mei before he died.
-Nagato: The guy never lost to anyone without plot weakening him some way. He mastered all elements so why didn't he use any of them? Him mastering all elements would allow him to make the same black jinton element like Obito since that's a Rinnegan power and he's not going to just learn Sage jutsus, Madara taught him all Sage Jutsus. He had to be crippled to limit him movements and he still soloed Jiraiya, 6 tail Jinchuuriki, Kakashi/Choji dad and co, Hanzo, Hanzo's forces and Danzo's anbu root forces, a ninja when he was an untrained kid, as an edo Tensei Kabuto used him to solo Bee and KM Naruto.
-Madara: Soloed Sai, Naruto, Sasuke, 9 Bijuus, random ninjas, Oonoki, Muu, SA ninjas from Gaara's division, Gaara, Oonoki, A, Tsunade, Mei, Bee, Gai, Edo Hashirama
_Minato couldn't have been Solo king since Nagato older than him and became solo king before him.
_Itachi couldn't have been solo King because Nagato was before him and Itachi died before Nagato so he couldn't have been after him either.