It was always a given that Aizen, no matter his genius, raw power, or the subtleties of his keikaku, would falter before the glorious might of the ever-evolving protagonist, because what sort of story would it be otherwise, anyway? However, since he was quite methodical in his ways, it took Ichigo a large section of his narrative timeline to build up enough strength to face him on equal ground. Not to mention how many other characters had to die in order for this to happen.
Even though Bleach has had two major antagonists after Aizen — Kugo Ginjou and Yhwach — the former has generally been considered the poster-boy villain of the story, in large part due to his charm and constantly deceitful ways. Is there any possibility that he could have won, had he acted differently in some way? As it happens, there might be.
10 By Preventing Ichigo's Birth
In the final arc, a flashback scene reveals that Isshin Shiba and Masaki Kurosaki accidentally meet each other while fighting White, the pseudo-Vasto Lorde that is released into Karakura Town.
When Tousen insists on removing the extra factors involved, Aizen asks him to stand down, claiming that the product of a Shinigami and a Quincy (with Hollow genetics somehow mixed in) would be of incredible research interest for him. He knows exactly what that product would grow up to become, but he allows it anyway.
9 By Using Hado #99
Sosuke Aizen has a preference for Hado #90, Kurohitsugi, or Black Coffin, probably because it is so dark and dismal (and the result is quite gruesome, which fits perfectly with his violent philosophy).
He employs it against Ichigo, replete with full incantation, only for the powered-up Substitute Shinigami to shatter the entire framework with a casual wave of the arm. Instead, Aizen should have thrown Hado #99, Goryuutenmetsu, at Ichigo — five gargantuan reiatsu dragons bursting from the earth would definitely have been problematic, even for Ichigo.
8 By Incapacitating Isshin Kurosaki
As a prelude to the earlier point, the main reason that Ichigo is able to actually defeat Aizen with his Hougyoku augmentation is that he suffers through several weeks of time-compressed mental training to extract the technique known as Mugetsu from his zanpakuto.
And this is possible since Isshin Kurosaki assists his son in reaching that meditative state, simultaneously keeping the Dangai from collapsing around them. Aizen fails to put him out of action in their earlier battle, instead opting to ignore Isshin, Urahara, and Yoruichi as worthless opponents.
7 By Killing Ichigo During The Ryoka Invasion
Aizen had ample amounts of time to get rid of Ichigo, considering the kid had absolutely no clue how Soul Society worked at the time. But then he goes ahead with his original plot (fake-crucifying himself and then trying to get the Hougyoku out of Rukia's gigai).
He gets the chance to complete the task when Ichigo cluelessly attacks him head-on, but his response is a minor slash across the belly, not nearly enough to finish off his quarry. What on earth did Aizen have planned for Ichigo anyway?
6 By Actually Finishing Off All The Captains (& Visoreds)
During the Fake Karakura Town battle, Aizen allows his Espada to weaken all the Shinigami Captains and Visoreds, entering the fray at the last minute to strike them all down in a series of fantastic moves. If his strategy really is to take over the Royal Realm, then there is no reason why he shouldn't have ended the military of Seireitei on the spot.
Further, he claims that he wants to kill Yamamoto Genryuusai but is distracted by Ichigo and then forgets about the old man. What if next time the Captain-Commander's bankai comes out to play? The Hougyoku is nothing more than a joke next to Zanka no Tachi.
5 By Making A Contingency For Urahara
While taking Isshin, Urahara and Yoruichi singlehandedly, Aizen morphs into his bizarre cocoon form, later erupting into a weirder butterfly. He doesn't notice that his intellectual nemesis had stealthily embedded a confinement kidou inside another, which is set to activate when his reiatsu level drops low enough.
Aizen, riding on the high of his recent triumphs, is stunned to discover this fact after Ichigo's Final Getsuga forces him to devolve. Such a silly outcome, especially because he knows exactly what Urahara is capable of.
4 By Being More Sneaky
For over a hundred years, Aizen hatches his devious schemes in absolute secrecy, divulging the core of his motivations to nobody except Ichimaru Gin and Kaname Tousen.
He gets Urahara and Tessai blamed for his Hollowification experiments on blameless Captains and Vice-Captains, with Shunsui Kyouraku vouching for his alibi (an aspect of Kyouka Suigetsu, of course). So why does he suddenly decide to disclose his intentions to all of Soul Society? If only Aizen had waited until the right moment.
3 By Directly Attacking Soul Society
Yhwach and his Sternritter don't hang around twiddling their thumbs — a few days after the initial invasion, they mount a massive attack on Seireitei, going all out from the get-go. Aizen, in comparison, retreats to his mega-palace, Las Noches, where he continues to concoct different lines of opportunity.
Imagine the ten Espada and their Fraccion on the offensive in Soul Society instead of in Karakura Town. The Shinigami would all be taken by surprise, which honestly is a key element in many victories.
2 By Maintaining Some Humility
Aizen is best known for his arrogant attitude towards everything else in existence. He believes himself to be the only being capable of seeing "how the world should be, and not how it is."
More importantly, he plainly mocks Ichigo on multiple occasions, calling him weak and useless, proclaiming that the Substitute Shinigami has only reached that level because he has allowed it to happen. Aizen simply cannot perceive one of the most basic principles of power: that the stronger person tends to win, and refusing to acknowledge that certainty is a part of his failure.
1 By Going Bankai
The most unexplainable thing about Aizen Sosuke is why he doesn't use his bankai even once. Does he have one? He obviously does because there is no way that someone who seeks the pinnacle of greatness would not have achieved at least this stage.
Is the shikai, Kyouka Suigetsu, really his bankai, and he's been lying all along? How could a secondary upgrade improve Perfect Hypnosis, which is perfect by definition anyway? There is much to understand about the complexities of Aizen's brain.