The Thousand-Year Blood War arc concludes Bleach, with Yhwach and his Sternritters playing the major antagonists. Ichigo and Soul Society are backed into a corner by these villains, whose power levels are only one aspect of the danger they represent.
In fact, the Sternritter prove themselves to be far more intelligent than their opponents on several occasions. Not all of them can be considered geniuses, obviously, but there are a few who can be classified among the smartest characters in Bleach. The Sternritter are ultimately wiped out, but only because the Shinigami receive overpowered upgrades towards the end of the arc.
NaNaNa Najahkoop
NaNaNa Najahkoop doesn't have the privilege of a story arc during the Thousand Year Blood War, but at least he's one of the smarter Quincies. Although the villain is naturally composed and alert, he hides his true nature under a veneer of overconfidence in order to trick his opponents into underestimating him.
NaNaNa's Underbelly, a technique that can deflate enemy reiryoku, is a reflection of his analytical nature. Then again, he's clearly not smart enough to know what will happen if he tries a sneak attack on Yamamoto.
BG9
BG9 understands that direct combat doesn't always result in the desired outcome, so it goes out of its way to threaten targets with the deaths of their family, as Omaeda learns when his sister is in peril.
BG9 combines physical strength with data collection, obtaining the latter with the help of wiry tentacles emanating from its body. Unfortunately for this Sternritter, it seems that Suì-Fēng brute force attacks are more than enough to overcome BG9's robotic intellect.
Giselle Gewelle
Giselle is a complicated character, in part due to her macabre Schrift, The Zombie, but mainly because it's nearly impossible to predict her next move.
The "undead" Sternritter has dozens of tricks up her sleeve, contingencies within contingencies, not to mention using both allies and enemies for her own benefit. During the second Sternritter invasion, Giselle nonchalantly converts Bambietta, Rose, Kensei, Matsumoto, and Hitsugaya, into her personal zombies.
Meninas McAllon
Meninas doesn't really care much for subtlety, but her directness is what makes her sharp-witted. Her situational awareness is displayed when she chides Bambietta for not covering all her bases (by killing every witness to her crimes instead of just one soldier).
Meninas doesn't come off as a tactical genius because her Schrift, The Power, is only meant to blow her opponents away with a single punch, but she keeps a cool head in the heat of battle, regardless. She is one of the few villains who doesn't deserve her unfortunate conclusion.
Robert Accutrone
Robert Accutrone chooses his words wisely when debating his opponents, taunting Shunsui just enough to keep the Shinigami captain on edge. The Sternritter proudly acknowledges Yhwach's invulnerability, a claim that is seemingly disproven when Yamamoto brings out his Bankai.
However, it is eventually revealed that Accutrone's initial assessment is accurate and that Shunsui—an extremely intelligent character himself—fails to analyze the situation as efficiently.
Shaz Domino
Although Shaz Domino is killed offscreen (and instantly), he devastates the Shinigami Research and Development Institute without expending too much mental effort.
This feat is especially impressive because of the SRDI, led by Mayuri Kurotsuchi, are well known for their scientific intelligence. On the other hand, Shaz prefers to fight using his reishi knives, a patently physical technique that doesn't rely much on strategy.
Royd Lloyd
A fan-favorite villain, Royd Lloyd is one of a set of twin brothers, each with very similar powers. That said, he is considerably smarter than his older sibling, Loyd, given that he survives both Kenpachi and Yamamoto's attacks (to a limit).
Royd's knowledge of the subject, namely Yhwach's personal history, grants him a level of intellect possessed by few Sternritters. In addition, Royd is capable of casting highly complex Quincy spells like Kirchenlied: Sankt Zwinger, forcing Yamamoto to unleash the totality of Zanka no Tachi to take him down.
Lille Barro
Lille is a terrifyingly powerful anime villain in part due to his so-called "divine" powers, pushing Kyoraku to the brink of defeat when he activates his Jilliel. Nevertheless, as the first Sternritter to be created by Yhwach, his combat intuition and judgment are off the charts.
There are a few occasions when Lille overtly displays consternation, like when Kyoraku attacks him with Karamatsu Shinju; however, he quickly regains his wits after evaluating his enemy's entire range of abilities.
Jugram Haschwalth
Jugram Haschwalth has a cautious personality, a feature that serves him well in nearly every scenario—the most relevant being his unrelenting suspicion of Uryu Ishida.
Yhwach exhibits no anxiety regarding his latest successor, but Haschwalth is far too shrewd to let his doubts go unresolved. The Wandenreich's second-in-command is as reliable and trustworthy as he is rational, explaining why he is given the honor of hosting the Almighty whenever Yhwach decides to sleep.
Askin Nakk Le Vaar
Askin demonstrates his wild-card tendencies almost as soon as he's introduced, making comments about his opponents' and allies' behavior with equal contempt. Yhwach promotes him to Schutzstaffel because he's satisfied with the Sternritter's skill level and overwhelming brainpower.
Askin's Deathdealing requires a complicated set of strategies for the Schrift to work properly; and yet, he can push the likes of Urahara Kisuke to the brink of death, implying that Askin lacks little in the IQ department.