Levi Ackerman is considered to be the strongest character by many fans of Attack on Titan. He was known as humanity's strongest soldier at the beginning of the series and is as powerful as titan shifters despite not having any titan abilities himself.
Unfortunately, toward the end of the manga, he almost died and wasn't able to fight nearly as well as he was in earlier arcs, but the fact that he still helped his comrades during the Rumbling proved how strong he was even in his weakest moments. Throughout the series, he fought a lot of strong characters, like Kenny and Zeke, but there were also many whom he didn't fight.
10 Oluo Was The Soldier In The Special Operations Squad Who Had The Most Kills
Being one of the Survey Corps' captains, Levi had his own squad since he was introduced in the series. His original squad consisted of Eld Jinn, Gunther Schutlz, Petra Ral, and Oluo Bozado. Out of the four soldiers, Oluo was the only one who had more kills than assists. He killed 39 titans and helped defeat nine others. Because Oluo looked up to Levi, would follow all of his orders, and do anything to protect him, there was no reason for the two to ever fight, but if they did, the master would definitely defeat his student.
9 Ymir Was The First Inheritor Of The Jaw Titan To Be Introduced
Ymir was one of the most mysterious characters at the beginning of the series. Eventually, her secret identity as the inheritor of the Jaw Titan was revealed. She was already a strong soldier and should have been in the top ten students of the 104th Cadet Corps, but when she transformed into the Jaw Titan to protect Historia, Reiner, Connie, and Bertholdt, her true power was shown. She died shortly after as she decided to give the Jaw Titan back to Marley in order to protect the warriors. Because Levi proved to be stronger than Porco, Ymir's successor who surpassed her, it's clear that he could have won a fight against her as well.
8 Connie Was The Eighth Strongest Soldier From The 104th Cadet Corps
Before joining the Special Operations Squad, Connie was the eighth strongest soldier from the 104th Cadet Corps. However, after fighting the titans who attacked Trost District with his friends, he decided to join the Survey Corps with them rather than the Military Police like he had always planned.
Connie always understood that he needed to follow Levi's orders, which is something that not all of the other members of the Special Operations Squad did. Because of this and the two of them being on the same side since they met, Connie and Levi never fought, but humanity's strongest soldier would have won if they did.
7 Jean Was The Sixth Strongest 104th Cadet Corps Soldier
Like Connie, Jean had also always planned on joining the Military Police after graduating from the 104th Cadet Corps, of which he was the sixth strongest soldier. But upon learning that his closest friend, Marco, was killed, he knew that he had to fight against the titans. This was very similar to Levi's reason for joining the Survey Corps. Even though Jean didn't always agree with Levi, he quickly learned that the captain had more experience in fighting the titans and came around. Levi would have easily beaten Jean if they fought.
6 Gabi Was One Of Marley's Best Warrior Candidates
When she was first introduced, Gabi was the strongest warrior candidate of her generation, with many people expecting her to inherit the Armored Titan. Due to her hatred for the Eldians on Paradis, it would have made sense for her to fight Levi. However, even though they both attacked people the other person cared for, they were never actually in a battle against each other. In fact, by the end of the series, they actually became close friends. Seeing how Levi was injured for the majority of their time together, Gabi probably would have been able to defeat him, especially considering her huge role in defeating the Mid-East Allied Forces.
5 Falco Was A Warrior Candidate Who Inherited The Jaw Titan
Another warrior candidate who most likely would have been able to defeat Levi was Falco. Though he wasn't that powerful in the beginning of the series, he had inherited the Jaw Titan by the time he met Levi properly, and also possessed characteristics of the Beast Titan.
Though the other inheritors of the Jaw Titan would have lost to Levi, Zeke was able to survive multiple battles against the captain and only lost to him when he accepted death to come his way. Since Falco was basically a mix between the Jaw and Beast Titans who could fly, it would have been hard for Levi to beat him.
4 Hange Was The 14th Commander Of The Survey Corps
Hange was one of the first friends Levi had made when he joined the Survey Corps, and over the years, the two got very close. Even though they often bickered and got annoyed with each other, they never actually fought. Hange had even saved Levi's life a couple times throughout the manga, but if the two of them had fought each other, Levi would have been the victor.
3 Keith Was The Survey Corps' 12th Commander
Before Hange and Erwin, Keith was the commander of the Survey Corps. Even though he was considered by many to be the worst commander of the military branch, he was clearly strong. If not, he wouldn't have had such an important role in the first place. He also became the head instructor of the 104th Cadet Corps after leaving the Survey Corps. Levi would have defeated him if they were to fight, but luckily they were always on the same side.
2 Grisha Was An Inheritor Of Both The Attack & Founding Titans
Even though Levi battled Eren and Zeke many times, he never fought against their father, Grisha. And despite the fact that Grisha was an inheritor of the Attack Titan for years, he rarely used his powers. When he did, it was to steal the Founding Titan from the Reiss family and give it to Eren. It's hard to tell whether Levi would have been able to defeat Grisha, seeing as how he defeated Frieda Reiss. And even though Eren and Zeke had more experience using their titan abilities than Grisha, Levi wasn't able to defeat either one of them on his own unless they didn't fight back.
1 Kruger Was A Marleyan Officer Who Inherited The Attack Titan
Grisha had inherited the Attack Titan from Eren Kruger, a Marleyan officer who was also the secret leader of the Eldian Restorationists. When all of his followers except for Grisha were injected with titan serum and sent to Paradis, Kruger attacked the other Marleyan officers, killing all of them. He then saved Grisha, who became his successor. Based on the one battle that Kruger fought in, he was the one Attack Titan Levi definitely would have been able to defeat.