The main antagonists in Bleach are the Hollows, with a mask-less offshoot known as the Arrancar taking center stage after the Soul Society arc. The strongest among these creatures are conscripted by Aizen into his private battalion, eventually forming the Espada of Hueco Mundo.
The Espada are known as much for their intelligence as for their overpowering strength, which is why they make such dangerous opponents. That said, not all Hollows are created equal—there is always going to be a distinction between those who are smart, and those who are not.
Nnoitra Gilga
Nnoitra's battle prowess and devastatingly powerful zanpakuto are certainly respectable, but that's pretty much the only positive feature of this Espada. He is violent, corrupt, shameless, vulgar, and not very smart at all.
His lack of IQ is displayed via his one-dimensional ambition: to be the greatest Espada (and, more importantly, to be stronger than Nelliel). Nnoitra cannot tolerate that a woman can be more powerful than him, and constantly tries to prove his masculine "superiority" by attacking Nel.
Yammy Llargo
Yammy is sorely lacking in the common sense department, especially in of fighting those who are clearly above his level. Ulquiorra constantly chides him for recklessly rushing into battle, but his desire for dominance is ever so slightly more than his fear of the #4 Espada.
In any case, Yammy doesn't quite need intelligence after he transforms from #10 to #0, officially becoming the most powerful of Aizen's Espada. His Resurreción form is massive and mighty, but also lends a comical air to the anime villain.
Aaroniero Arruruerie
Aaroniero Arruruerie is a Gillian-level Espada, making him relatively weaker than the rest of his cohort, but he compensates for it with cunning strategies.
During his fight with Rukia, he makes a calculated guess as to what would emotionally and psychologically hinder his opponent the most, reaching the right conclusion by mutating into her beloved mentor, Kaien Shiba. Aaroniero's plan would have worked had it not been for Rukia's lucky final strike.
Grimmjow Jaegerjaquez
Grimmjow's biggest weakness is his inability to control his impulses, leading to overly brutal fights whenever he faces a target he believes to be worth his time and effort. However, he is quite cautious in the throes of battle, as seen when he shrewdly locates and takes advantage of Ichigo's drawbacks.
Grimmjow eventually sides with the Shinigami, artfully exploiting Urahara's hidden path into Askin's Gift Bereich and finishing off the smartest of Yhwach's Sternritter in an instant.
Zommari Rureaux
Zommari is known for his meditative capabilities, which help him maintain a steady emotional state in the most intense of battles. He manages to push Byakuya Kuchiki to his limits—forcing the Shinigami to bring out his bankai.
A combination of Senbonzakura Kageyoshi and a powerful Kido technique known as Danku seals Zommari's fate. It's unfortunate that an Espada with his level of skill and perception is paired with an overpowered opponent.
Baraggan Louisenbairn
Baraggan Louisenbairn used to be Hueco Mundo's God-King before Aizen dethrones and humiliates him in front of his "subjects". Although he reeks of arrogance, the #2 Espada is a wily fighter, keeping Suì-Fēng, Omaeda, and Hachigen Ushoda on their toes for a significant portion of their battle.
Baraggan's situational awareness is impeccable, considering he's among the first to figure out that the Shinigami had switched the real Karakura Town's Jureichi with a simulation, realistic though it appears to everyone else.
Coyote Starrk
Stark pretends as though he doesn't care about anything or anyone, but this disionate attitude is stripped away when he fights Shunsui. His lackadaisical exterior is, in reality, a mask that hides the total extent of his reiatsu; not to mention, he never misses a single detail during battle.
Shunsui is shocked that the #1 Espada is able to counter his complex Shikai by mere observation, a sense of intuition the Shinigami acknowledges and respects. Starrk is probably the most heroic villain in Bleach.
Szayelaporro Granz
Szayelaporro Granz is known for several incredible inventions, such as Medicinal Fracción—which allows him to restore his health by eating his subordinates—and Rokureichu—tiny bugs that absorb and transmit data on his enemies.
Szayelaporro exhibits the classic intensity of the "mad scientist" trope, but it must be noted that he pales in comparison to the powerful Bleach Captain Mayuri Kurotsuchi, who stretches the limits of science to its most horrifying extremes.
Tier Harribel
Tier Harribel tempers her emotions with logic and practicality, seeing no purpose in leaping into a fight without evaluating all the potential outcomes beforehand. Further, Harribel can discern minute fluctuations in reiatsu, especially if they are based on emotional change.
Her insight into others' behavioral patterns helps her take opponents down with a patently un-Espada-like finesse, compelling Captain Hitsugaya to go all out with Daiguren Hyorinmaru.
Ulquiorra Cifer
Ulquiorra dismisses humanity as a whole, claiming that their need to express themselves is one of their greatest failings. Regardless of his biases, he is a terrifying combatant exactly because he feels nothing for himself, let alone anyone else.
Ulquiorra deduces that Ichigo's reiatsu can theoretically turn into a severe threat, taking a great interest in the young Shinigami as a result (but also to keep an eye on Ichigo's movements). Aizen himself recognizes Ulquiorra's towering intellect, explaining why he sends the #4 Espada to the Human World in the first place.