As a pretty standard shonen anime, Bleach is known for its often exaggeratedly powerful characters, whether villains or heroes. The Shinigami have a variety of Zanpakuto, each with a unique set of abilities, while Hollows, Espada, and Quincy possess specialized weaponry of their own.
As such, it's hard to imagine that Bleach characters could lose to benders from Aang can manipulate all four elements while activating the logarithmic effect of the Avatar State.
Aang Could Defeat: Ichigo Kurosaki
Ichigo is technically the most powerful character in the story, but that doesn't mean he wins every battle. In fact, the inexperienced Shinigami is no match for the experienced Airbender who also possesses the wisdom and knowledge of every Avatar in existence.
Zangetsu's Getsuga Tensho packs a powerful punch, but it's not something that Aang's bending talents can't handle. Most importantly, Ichigo wouldn't be motivated to go all out against someone younger than him, which is only inviting defeat.
Aang Would Lose To: Orihime Inoue
Orihime Inoue possesses an unclassifiable power that "treses into God's territory." Known as Phenomenon Rejection, Soten Kisshun allows her to basically negate an event after it has occurred, thereby making it so that it never happens in the first place.
Although she doesn't exactly flaunt this technique around, doing it would have actually helped save so many lives, though. In any case, Aang's Avatar State is powerless Soten Kisshun, unless he can muster up enough life aura to cancel Orihime's attack.
Aang Could Defeat: Kugo Ginjo
Ginjo's Fullbring powers are impressive and are clearly capable enough to wresting control of Ichigo's technique and taking it all for himself. Unfortunately for him, the latest Substitute Shinigami unlocks the full force of his reiatsu when Rukia impales him Urahara's patented spirit pressure blade.
If Ichigo could dispatch Ginjo with such ease, then the battle would be a cinch for Avatar Aang, who probably doesn't need more than one or two bending styles to win in this scenario.
Aang Would Lose To: Ishida Uryu
Ichigo and Uryu begin the story at a similar level of strength and conclude it the same way. The only difference is that Uryu takes fighting as seriously as any artform, while Ichigo prefers to hack and slash, relying on his reiatsu to win the day.
The Quincy is going to prove a huge thorn in the Avatar's side, more so because of his preferred choice of attack: long-range, which is a lot more efficient against Aang than physical combat.
Aang Could Defeat: Sado Yasutora
Sado Yasutora is the weakest member of Team Ichigo, but his two techniques — Brazo Derecha de Gigante and Brazo Izquierda del Diablo — are not easy to dodge (or survive if the target isn't very powerful.)
This is not going to be a concern for Aang, whose speed is augmented immensely by his Airbending, which, when combined with defensive Earthbending and offensive Firebending spells triumph for the young Avatar against a forgotten Bleach side-character.
Aang Would Lose To: Byakuya Kuchiki
Byakuya's experience alone ranks him among the more impressive Shinigami of the Gotei 13, an astounding feat given the talent of some of the other Captains.
His shikai is trained to pointillistic perfection, but in case it doesn't work against the Avatar, there's always Senbonzakura Kageyoshi. Byakuya's bankai is a force of nature by itself, excluding his strategic brilliance and tactical use of kido as an element of surprise. Aang simply can't match the Squad 6 Captain in combat.
Aang Could Defeat: Toshiro Hitsugaya
Toshiro Hitsugaya's reiryoku is beyond measure, so much so that Gin believes the former will become one of the most powerful Shinigami of all time.
While this potential fate seems likely, Aang can comfortably take advantage of. Another point in favor of the Avatar is that he can Waterbend any ice that Hyorinmaru throws in his direction, negating Hitsugaya's attacks as they are far above absolute zero, unlike Rukia's.
Aang Would Lose To: Yamamoto Genryusai
Yamamoto Genryuusai, the Captain-Commander of the Gotei 13, doesn't fight much in the story because of one reason: any battle he enters would compulsorily end in his victory. In fact, everything would be literally finished if the old man brings out his bankai, against Avatar Aang.
The sun-based techniques of Zanka no Tachi are theoretically capable of evaporating Yamamoto's surroundings for miles, being approximately 15 million degrees in temperature. Aang could Waterbend an ocean, but there's no win for the Avatar here.
Aang Could Defeat: Bazz-B
Bazz-B is nowhere nearly as overwhelming as Yamamoto, despite his claims to the contrary. His Burner Fingers are quite brutal, though, and clearly capable of piercing strong individuals like Hitsugaya and NaNaNa.
This opponent might require Aang to trigger the Avatar State in order to ensure victory, but doing so is certain to end the fight in record time. Bazz-B isn't going to be able to fend off a medley of bending techniques simultaneously.
Aang Would Lose To: Sosuke Aizen
Unlike Fire Lord Ozai, who initiates altercations and fights in public spaces, Aizen works in secret, slyly manipulating the inhabitants of Seireitei for decades before his defection.
It's possible for Aang to hide from Kyoka Suigetsu, but the real problem, in this case, is Aizen's incandescent reiatsu. The Avatar's best shot will fail to break through Kurohitsugi #90 performed at full strength; and if Aang dies in the Avatar State, then he ends the cycle forever.
