Alright, let's talk crazy knights. The one that instantly comes to mind is The Knight of the Sorrowful Countenance himself, Don Quixote. Calling him just 'crazy' feels a bit reductive, but Cervantes absolutely built the archetype of the delusional knight-errant. His madness is a romantic, tragic engine for the whole story. He's not malicious, but his complete break from reality makes him an accidental antihero—he genuinely believes he's righting wrongs, while causing chaos. It's less about dark brooding and more about poignant, misguided idealism. That contrast between his noble intentions and the ridiculous outcomes is the core of his antihero status.
A much darker, more modern take would be Ser Gregor Clegane, 'The Mountain,' from 'A Song of Ice and Fire.' He's less a knight in the chivalric sense and more a brute in knight's armor, but his titles and status are part of the universe's brutal irony. His 'craziness' is pure, sadistic rage and violence, amplified by magical torment later on. He's an antihero only in the sense that he's a monstrous force used by the 'villains' we also follow; you're never rooting for him, but he's a fascinating instrument of terror in a world where knightly vows are constantly shattered.
For a fantasy deep cut, I'd throw in Ser Brendel Byrne from some interpretations of the Thomas Covenant universe—or even Covenant himself if you squint, though he's not a knight. But the trope of the mentally shattered, obsessive knight fighting a war they can't truly comprehend is a rich vein. Quixote remains the blueprint, though. The madness isn't just a character flaw; it's the lens that critiques the entire genre of knightly tales.