Look, my country is messed up so have some salutary shitposting while I attempt to cope with health/life/everything.

No particular order, define "best" as you please - I mean "somewhat plausible-sounding-ish and fun to read about."

Rated from VERY SCARY, Scary, unconcerned in terms of, hrm, threat level.

Baru Cormorant from Seth Dickinson's books: Scary.
High INT, low WIS. Scary, but doesn't achieve VERY SCARY due to too many emotional vulnerabilities.

Hanse Davion and Ulric Kerensky from BattleTech. I just don't want to be in the same universe they're scary. I'm MORE scared of Hanse Davion than Thrawn because I get the possibly illusory sense that Thrawn is civilized as a default and the vibe I get from Hanse Davion is that civilization, cruelty, courtesy are all just tools, he will do whatever it fucking takes to burn you to the ground if that's the way to win.

Hanse Davion: VERY SCARY
Ulric Kerensky: Scary, but also, clanner honor.

Ari I and II from CJ Cherryh's Cyteen: SCARIEST.
Justin: unconcerned, honestly, give him research funding and pizza and Grant and he's happy, he'll leave you be.

Conrad Mazian from Downbelow Station: Scary and they lucked out he was undone.

Flamme from Frieren: SCARIEST. I almost rate her

Scary not because she's not a terrifying genius but because she has ironclad ethics. Probably the single person on this list I'm MOST afraid of except she's also UNAMBIGUOUSLY GOOD. So she's a rarity: a female chessmaster (or anyway, they're incredibly rare in English-language USAn sf/f) and unambiguously a "good guy."

Lelouch Lamperouge from Code Geass: Scary. Possibly shading into SCARIEST if you add the mind control, but make Nunally cry and he segfaults.

Vladilena Milizé from 86, and how. Scary.

Thrawn: ???
Thrawn from Star Wars Extended Universe is frequently cited but I bought the Timothy Zahn book where he first? appears? extendedly? as a military? genius? for Kindle and I refuse to use Kindle anymore so I'm going to have to suck it up and buy a print copy if I can even remember the title. Anyway, I haven't read books with Thrawn doing stuff so I can't comment further.

Lord Vetinari from Terry Pratchett's Discworld: SCARIEST.
Doesn't generally come up in these discussions because bureaucracy is "boring" and Vetinari wasn't a main character in any of the Discworld books I read. (I binged them for a couple months twenty years ago, then never went back, sorry.) He's a fucking EFFECTIVE BUREAUCRAT. I don't mess with those.

Maomao from Apothecary Diaries. Unconcerned ONLY because she's easily bribed with bezoars. :3

Miles Vorkosigan: Scary.
Honestly one of the most plausible military geniuses BUT ALSO a disaster for all his subordinates. I don't want to be within a galaxy radius of him.

Hiruma from Eyeshield 21. Unconcerned mainly because I don't have ANY involvement in Japanese high school instantiations of American football and FORTUNATELY his domain of interest is VERY SPECIALIZED. :)

Both Seondeok and Misil from The Great Queen Seondeok: Scary to Unconcerned.

Laurent from C. S. Pacat's Captive Prince books. Unconcerned mainly because Good But Not Nice.

Red from The Blacklist: Scary by way of UNHINGED.

Lady Char from Mobile Suit Gundam: Witch from Mercury: Scary. Sorry, I can't focus my eyes enough to dig her name out of the walls of text on various wikis. :]

Beth Harmon from The Queen's Gambit. So very unconcerned. I'm not a chess player. I don't have anything to worry about.

Ikari Gendou from Neon Genesis Evangelion: SCARIEST and also worst dad of the millennium.

Ted Lasso and Keely (sp?) from Ted Lasso: unconcerned, but could well become Scary in an AU. Somewhat uncommon double example of people who are brilliant socially AS WELL AS being good people; Ted Lasso or Keely with that skillset using their powers for EVIL would become horror rapidly.

Asshole Protagonists from K. J. Parker's books are generally Scary. Asshole Genius is pretty much the shtick.

That Guy from The Usual Suspects. Probably SCARIEST but I haven't watched that movie in two decades.

There are going to be comicverse examples that I'm just not familiar enough with to comment further. /o\ Or multiple characters from Re: Zero but thinking about details is too traumatic (complimentary).

Cotton handspun single from combed top, a "completed" bobbin. I'm spinning threadweight so I don't...feel the need to "fill" the bobbin even halfway (for a planned 2-ply).

I do think I'd probably have a more pleasant time spinning cotton and silk if I had a dedicated treadle wheel for them, someday; but the wheel I own works. :3

(The background art on the wall is a poster of Wonder Woman artwork by Nen Chang.)