Hajime Isayama has never shown hesitation when it comes to killing off characters in Attack on Titan. He has written many unexpected character deaths, and this is a big reason why the series is such a huge success. Ever since the revelation of how Titans are made though, there have been numerous debates.
Fans of the series didn't originally know that the powers of special Titans could be given to normal humans. As a result, once the revelation was made, debates surrounding which other Attack on Titan characters could have inherited the power of the Titans quickly began to take place.
6 Erwin Smith
The name Erwin Smith often pops up while discussing candidates who should become Titans. Being the leader of the Survey Corps, Erwin was an obvious choice. He was smart and a leader of men, who was able to rally his troops and make them march towards certain death to provide humanity with a chance to take down one of the strongest Titans. Erwin Smith died during the fight against the Beast Titan. However, before dying, for a minute it seemed like Levi would use the Titan serum to revive Erwin and let him consume Bertholt.
It is apparent that Erwin's leadership skills were pivotal to the Survey Corps and having an "unkillable" Titan as their leader would have boosted their morale even more. Additionally, it would have also enabled Erwin to learn about the secrets inside Eren's basement. Eventually, though, Levi decided against using the serum on Erwin and instead used it on Armin instead.
5 Mikasa Ackerman
Mikasa is one of the main characters in the series and has been protecting Eren since they were kids. She's been very supportive of him and has stood by him even in the toughest of times. However, it wasn't always the correct choice. Mikasa is an incredible fighter, and it would be fair to say that she's second only to Levi when it comes to fighting Titans.
Mikasa could have become the Female Titan and helped Eren in the war against the Marley. Annie, who was also a good fighter, was able to outmaneuver the Survey Corps soldiers and other Titans. With this in mind, it seems fair to say that Mikasa would have been much stronger than Annie, given that the former is the better fighter.
4 Gabi Braun
Ever since her introduction, Gabi has been criticized by a lot of fans because of her bad attitude. Many fans disliked how she behaved, but it's easy to forget that she's just 12 years old. She's a very talented fighter for her age though, and, with her efforts, she earned the spot of a Warrior Candidate.
For a young kid, Gabi has a very strong resolve. Throughout the series, she has been seen stating that she will definitely inherit the Armored Titan. Gabi's resolve is extremely commendable and, after the Paradis' attack on the Marley, it only gets stronger. It is plausible to assume that if Gabi had become the Armored Titan, it would be incredibly difficult to stop her.
3 Levi Ackerman
Levi Ackerman is the strongest non-titan character in the entire series. Born in a place that was filled with crime, he had a pretty rough childhood. His struggles only made him stronger though, not just physically, but mentally as well. When he was recruited by Erwin, Levi showed signs of promise, and, in a short period, became one of the most famous names in the Survey Corps.
During his confrontation with Kenny, Levi got his hands on the serum and could have used it to turn himself into a Titan, but he chose not to. If he had decided to seize the power, however, there's no doubt that he would have easily been the strongest Titan; at least in terms of fighting abilities, anyway.
2 Kenny Ackerman
Kenny Ackerman was Levi's uncle and taught him how to fend for himself. Kenny was not a nice person by any means. In fact, he was known for killing MPs, which led to his moniker, "Kenny The Ripper."
After working with Uri Reiss, Kenny managed to curb his killing, but he also began to lust after the power that the Reiss family possessed. If Kenny had become a Titan, he would have been an even better villain. It would have been amazing to see Kenny fight against Levi while in Titan form.
1 Hange Zoe
Hange Zoe took over the leadership of the Survey Corps from Erwin Smith. She's a very intelligent character, who has helped the Survey Corps to learn more about the Titans. This, in turn, has resulted in the organization becoming much better at taking Titans down.
Hange might come across as eccentric at times, but there can be no denying that she's better at analyzing Titans than anyone else. She's very brave too, as putting her safety at risk does not seem to faze her. If Hange was turned into a Titan, she could have easily learned a lot more about Titans and their abilities. It is possible that with enough time, Hange could have even figured out a better way to stop them. However, Isayama clearly had other plans for her.