I'm looking for manga with a protagonist who is (at least mostly) calm and collected, and has amazing analytical or strategic/tactical thinking abilities. Examples of the kind of MC im looking for are Kanme from Darwin's Game, Akiyama from Liar's Game, Tatsuya from Mahouka Koukou no Rettousei, Tomu from Gamble Fish.

Any recommendations you could give me would be so appreciated! MC can be male or female. Any genre, except yaoi/shounen-ai.

Dr. Frost
Chang Ge Xing
Munakata Kyouju Denkikou (& sequel)
Ubume no Natsu (& sequels)
Sangokushi (Kongming is one of many protagonists, and he appears about halfway through. The whole manga is about battle strategy etc.)
Yuureitou (Testuo, not the protagonist but almost as important and far more interesting)

I also thought of Gosick (Novel), but I haven't read them, I've only seen the anime (and I don't think there is a manga). And she's not really the protagonist.

Koi da no Ai da no has its main character, a female lead, like this, although the story is centered on romance (since it's shoujo after all).

The Code Geass series may be of some interest to you if you haven't already read them.

Other series are:
Origin
Bokura no Kiseki
ES from what I remember has a pretty calm protagonist
Homunculus

Currently getting my life together~ (unsuccessfully)

It's a light novel but Altina the sword princess's MC is litterally a good strategist.

Chang ge Xing is incredible. A somewhat realistic historical drama about the main character who escapes the downfall of an empire and uses her intelligence (war tactics, general genius) to carry out her plans for revenge etc.

Art is gorgeous and I love the characters.