Aizen used to be the most broken villain in Bleach, and then Yhwach showed up.
Throughout the first half of the Bleach series, Aizen was the main enemy of Soul Society. Whether it was his sheer intellect or his overpowered abilities, most Shinigami didn't stand a chance against him. After his eventual defeat and imprisonment, Aizen only grew in power.
During the Thousand Year Blood War, Yhwach took over the role of the main villain. He was so powerful that Ichigo had to work alongside Aizen just to destroy him. The question remains whether or not Aizen could also pull off that feat by himself. It's dependant on how he deals with The Almighty.
Can Aizen defeat Yhwach in Bleach? Here's a look at what both men bring to the table
Aizen and Yhwach had a brief encounter during the climax of the Thousand Year Blood War. Tite Kubo made it clear that Yhwach is the strongest character in the Bleach series. Aizen would probably lose 99.99% of the time. Regardless, there is a very unconventional way that he can deal with Yhwach.
Aizen completely breaks the power scale for Bleach
Ever since Aizen powered himself up with the Hōgyoku, he was granted immortality. Aizen cannot be killed by normal means, since he can regenerate himself. He can only be weakened to a point where a Kidō spell would bind him in place, which is how Ichigo and Urahara defeated him.
Aizen is so powerful that he can easily defeat several members of the Gotei 13. His physical and spiritual power completely surpasses the Shinigami. Yhwach even declared him a Special War Power just for his large amount of Reiatsu. Most characters would be crushed underneath Aizen's presence.
Of course, the real trump card is the Kyōka Suigetsu. It's among the most dangerous Zanpakutō in the Bleach series. Whenever a target is under the effect of his Shikai, Aizen can fully control their five senses. Even a god like Yhwach can be tricked by this technique.
The almighty power of Yhwach
The most fearsome aspect of Yhwach is The Almighty, a special ability that allows him to see into multiple futures. He can also change those futures into something that benefits him directly. For example, if Yhwach somehow dies, he can prevent that from happening as long as The Almighty is active.
Whenever he looks into the future, Yhwach will also have power immunity to whatever he sees, including Shikai and Bankai. Ichibē Hyōsube found out the hard way after Yhwach survived the Futen Taisatsuryō, a move that was meant to completely destroy him.
Chapter 630 states that Yhwach would lose his power if Jugram Haschwalth dies using it. Whenever his right-hand man wields that power, Yhwach also becomes defenseless in this state. It should be noted that Yhwach can use other Sternritter powers, but he rarely does so.
In a normal fight, Aizen doesn't have a way to defeat Yhwach. The latter cannot only see into the future, he can alter it to his advantage. Yhwach is nearly invincible as long as his Almighty powers are active. He can even rewrite a future where he dies by resurrecting himself.
Aizen does have immortality, but he can still be incapacitated through other means. Back in Chapter 684, Yhwach engulfed him in darkness, seemingly defeating Aizen in the process. With that said, there is one path to victory that Aizen can take, but it won't be in a one-on-one match.
Realistically, the "easier" route would be to kill a defenseless Yhwach while Haschwalth is using the Almighty. Kyōka Suigetsu could set up illusions and distract Haschwalth while Aizen takes care of Yhwach elsewhere.
Note: This article reflects the writer's personal views.