The number of fights in Bleach is almost incomputable — in fact, the number of active characters themselves are up in the dozens. As such, the story has been broken into several arcs, based on the overarching theme as well as the events that shape Ichigo's path. These digestible fragments are not exactly focussed, but provide a general base over which the framework of narrative progression has been constructed.
Each arc contains its own series of battles, mostly interspersed with the relevant backstories and motivations, but all of them conclude with one of the several major showdowns that take place across Bleach. Well, nearly all of them, in any case.
10 Agent Of The Shinigami — Ichigo Vs. Renji & Byakuya
This was a crucial fight for Ichigo because it was the first time he had faced an opponent whose level was leagues beyond his own. He took care of Renji surprisingly easily, considering the fact that he had received his zanpakuto very recently (and hadn't even acquired its name at that point).
However, he was absolutely no match for the hypersonic powerhouse that is Byakuya Kuchiki. The Squad 6 Captain calmly snapped Ichigo's sword in half before attempting to do the same to him.
9 Soul Society: The Sneak Entry — Yoruichi Vs. Byakuya
Ichigo had beaten several characters by this time, including the likes of Madarame Ikkaku and Kenpachi Zaraki, and hadn't quite healed from his battle against the latter when he tried to steal Rukia away from under the watchful eyes of Kuchiki Byakuya. This time, though, the furious wielder of Senbonzakura did not take on the orange-haired ryoka, but his own mentor from a long, long time ago.
Shihouin Yoruichi and Byakuya exchanged a few words of smack, but only one of his statements was proved right. The ex-Captain of the second division showed her pupil that he would need more than a hundred years to get near her skill tier.
8 Soul Society: The Rescue — Ichigo & Renji Vs. Aizen
To be fair to the heroes here, both Renji and Ichigo were severely wounded from their respective fights against Byakuya when they discover the real evil behind all the events that had occurred for the past year: Aizen Sosuke.
Adrenaline surging through their veins, bolstered by their earlier victory, they arrogantly plan a combined attack against their latest enemy. Unfortunately for them, Renji's Zabimaru provides zero distraction, and even if it had, Aizen would have still blocked Ichigo's Tensa Zangetsu with nothing more than his index finger.
7 Arrancar: The Arrival — Ryuuken Vs. Uryuu
The fight here isn't real, in that it is a kind of sparring match between Ishida Ryuuken and his son, Uryuu. The boy barely kept up with his father's ridiculously overpowered Gintou, until realizing that his opponent seemed to be losing strength.
The last-ditch attempt to turn the tides in his favor failed miserably because Ryuuken effortlessly displays his awe-inspiring reiatsu by striking Uryuu with one of his reishi arrows. Interestingly, this action concludes the battle, while simultaneously restoring the young Quincy's lost Letzt Stil.
6 Arrancar: The Hueco Mundo Sneak Entry — Ichigo Vs. Dordoni Alessandro Del Socaccio
Ichigo flatly rejected Dordoni's demands to trigger his bankai, as he had intended to conserve his spiritual pressure for the serious villains (like Ulquiorra and Grimmjow). The Privaron Espada showed contempt for what he believed to be the Shinigami's arrogance and used Nelliel to activate Tensa Zangetsu.
After this point, Ichigo gained the upper hand quite decisively and obtained a resounding triumph by adding his hollow mask to the mix. To summarize: Dordoni got one-shotted.
5 Arrancar: The Fierce Fight — Ichigo Vs. Grimmjow
Both characters had been spoiling for a fight ever since their encounter in Karakura Town, and their wish was granted at the end of this arc. Grimmjow mocked Ichigo for not fighting with a predatory mindset since he believed that any form of combat compulsorily required a murderous motivation.
A blur of back and forth exchange occurred, finally culminating in the 6th Espada releasing Desgarrón, his set of terrifying blue claws that are proportionate to the force exerted by Hollow-enhanced Tensa Zangetsu. At the last minute, Ichigo was reminded of why he came to Hueco Mundo in the first place and utterly vanquished Grimmjow.
4 Arrancar Vs. Shinigami — Kenpachi Vs. Nnoitra
Before Ichigo was cruelly crushed into oblivion by Tesra Lindocruz, Kenpachi dispatched the Fraccion in a single strike, terrifying the latter's master, Nnoitra Gilga. But only temporarily, because the 5th Espada was no weakling, certainly not with the might of Santa Teresa behind him.
The pair appeared to have no advantages over either of their opponents. To be honest, Nnoitra started gaining some serious ground after bringing out his extra praying mantis arms. Kenpachi was disappointed that he was forced to resort to kendo, which he hates the idea of for some reason, but at least it finished the job.
3 Arrancar: Downfall — Ichigo Vs. Aizen
The last confrontation between Aizen and Ichigo was an explosive celebration of power, each of them rising higher and higher, back to back, so as to destroy the other. Nevertheless, the hero had just undergone a three-month meditation crash course under the guidance of his dad, Isshin, and learned the true secret (and sacrifice) that lay behind the Final Getsuga.
Aizen couldn't accept that "a mere human" was capable of rising beyond his level, but his nuclear bomb cero barely singed Ichigo's left arm. The substitute Shinigami accepted the outcome of his destiny and empties every drop of reiatsu into Mugetsu, permanently ending Aizen's reign of terror.
2 The Lost Substitute Shinigami — Ichigo Vs. Ginjou
At first, Ichigo really believed that Kuugo Ginjou wanted to be his friend to help him regain some semblance of dignity after losing his Shinigami status post the Aizen debacle. They discussed their individual experiences as Soul Society substitutes, but neither of them managed to agree on who was right.
In the end, Ginjou's specialized bankai did not fare well against Ichigo's reinstated reiatsu. Interestingly, he was one of the only two characters that the protagonist put to death, the other being Yhwach.
1 The Thousand Year Blood War — Ichigo Vs. Yhwach
Yhwach, the Quincy King, had vacuumed up the Soul King into his anatomy (which, when mixed in with the Almighty, made him nigh-invincible). Therefore, Ichigo, Renji, Aizen, and Uryuu had to put their heads together and make a watertight plan to bring him down.
All this while being painfully aware that a single error or a missed opportunity or incorrect syncing would doom the whole world. Aizen pretended to be Ichigo, Uryuu pierced Yhwach with Still Silver, and Ichigo pounces on the final villain, halving him with his eroding bankai.