This question is a completely opinion based. Kubo has never stated that any character is the strongest in the series. Besides, there are a wide variety of powers within the Series of Bleach so it would be difficult to determine the one with the most strength as that can have several meanings.
- Genryūsai Shigekuni Yamamoto stated while fighting Sōsuke Aizen that there hasn't been a Shinigami born in over a thousand years who could displace him as Commander of the Gotei 13, thus admitting himself to be the strongest Shinigami.
- Aizen also admitted that he had created Wonderweiss Margela for the special purpose of sealing Yamamoto's Zanpakutō because Ryūjin Jakka was too powerful for him to win against Yamamoto. The Commander also survived an explosion of his own Zanpakutō.
- Later in the series, Yamamoto is defeated and killed by Ywach the Emperor of the Vandenreich a group of very powerful beings with Quincy abilities.
- Shunsui Kyōraku is now the Commander of the Gotei 13 following Yamamoto's death.
- Kyōraku, the new Commander of the Gotei 13, has stated that the Royal Guard Division is a small group of Shinigami who are more powerful than the members of the Gotei 13 combined. They are all former Captains who have been promoted to that division.
- Retsu Unohana has been revealed as the first ever Kenpachi and is thus one of the strongest fighters in the Bleach universe, continually bringing Kenpachi Zaraki to the brink of death before healing him. This is strength in multiple areas.
- Zaraki killed Unohana in combat and during the fight she continually insists that he is stronger than she ever was and has just been holding himself back over the years.
- The Espada are considered, by Aizen, to be the most powerful Arrancar or Hollows in Hueco Mundo. He initially stated that he would not have to lift a finger to defeat the Gotei 13 however, the Shinigami and Visoreds managed to defeat the Espada in combat, suggesting he may have exaggerated their power levels.
- Yammy Llargo is said to be the Cero (0) Espada, or the top ranked only when he was in his released form. He was, however, defeated by two Captains.
- Coyote Starrk is the Primera Espada and thus, considered to be among the strongest. He defeated two Captain-level Shinigami and faced off against four in total before being cut down by Kyōraku, Yamamoto's eventual successor, in single combat. This can also suggest Aizen misjudged power levels.
This is a list of some of the more powerful characters in Bleach and evidence which supports why they can be considered as such. However, this issue is entirely subjective until Kubo confirms which character he created is the strongest. In the series, many characters claim they are the most powerful due to their ego or for other reasons.