Sheogorath, Daedric Prince of Madness from Elder Scrolls. Under all the memes about his drug addicted cat form and cheese, lays the most clever and possibly most powerful of the still living gods of the Elder Scrolls universe. His madness gave him "inspiration" to create music, and used his trickery to deceive and outplay all other Daedra in their own fields, and at one point had to have all Daedric Princes team up to render him to his current maddended self.

Jevil, secret boss of Deltarune chapter 1. Still my personal favorite secret boss, he believes himself to be truly free despite being trapped in a dungeon within a false world of imagination. But he knows the prophecy, and eith so much being set in stone, he sees it as an excuse to do whatever he wants, knowing no matter what he does, it doesn't matter, in his mind the only true form of freedom.

Joker from DC. do I really need to go over how the most overexposed and popular villain in comics for 80+ years represents this? From his actually interesting stories and speculation, him almost knowing he's a frictional character so that he has no need to feel guilt or shame against the countless nameless corpses he leaves in his wake. He may have just been given the true nature of his world due to his madness, has "super sanity" of some sort, and straight up knows he's basically immortal.

Alice in Wonderland in general. One of the core themes in the original story and all adaptations is how carefree and cheerful the residents of Wonderland are despite living in a world of nonsensical chaos, most represented by the Cheshire Cat.

(Twisted/Interesting trope) Madness is true freedom/wisdom/power