Aizen's Bankai can be too overpowered for other Bleach characters to even see them properly.
The reason AIZEN never uses his BANKAI – ODBS
Tite Kubo has always been very vividly imaginative about how many of the characters in Bleach are. Their powers, their Bankai, and their visually stunning expressions make up some of the best characters. While his streak may not be at the top of the Big 3, it still stands to be a part of it. Additionally, villains like Ulquiorra, Yhwach and others are made to stand out.
The only villain that successfully stands as not only the best Bleach villain but also at the top among anime-only villains is Aizen. His skills and smarts make him a formidable foe that many cannot fight. Despite how long the character has spent in the series, his Bankai remains a mystery and there may be a good reason for that.
Throughout Bleach's run, Aizen's Bankai has been a mystery that no one has been able to uncover. Tite Kubo's hand has been hard to hide the power that the villain has and maybe the reason behind it is more than just the power it gives him. As suggested by a Reddit user, Aizen's Bankai could be the ace up Tite Kubo's sleeve.
