Light Yagami.

Because he is, in fact, quite dumb.

Log Horizon's Shiroe, at least in Season 1 I haven't seen anything newer. Its not that he is an absolute genius, but his plan to get all of the guilds working together while trapped in the world was really believable, clever, and it made complete sense as soon as it was revealed

What is the difference between a smart MC and a genius MC?

How smart would a character have to be to be considered a genius?

If I go with my own interpretation - that someone who is smart enough can be considered a genius - rather than the show having to explicitly label a character as a "genius", then my pick would be Aqua from Oshi no Ko.

Edit: forgot to explain what makes Aqua's intellect believable:

We were shown his thought process / inner monologue time and again, and it always sounds reasonable. Also, the people who he had to outwit never get dumbed down to move the plot a certain way. And yet his wit still managed to surprise me from time to time, proving his smarts.

(But to be fair, he's a grown man in a child's body, so he may not actually be a genius when compared to others in his age group.)

Also, just because someone is a genius does not mean that they are infallible or perfect in everything. In fact, them having flaws just make them more realistic.

Honourable mentions:

• Shirogane and Kaguya from Kaguya-sama (mentioned to have genius-level intellect / achieved gifted-level test scores in the anime IIRC)
• The trio from The Promised Neverland (mentioned to have genius-level intellect / achieved gifted-level test scores in the anime IIRC)
• Maomao from The Apothecary Diaries
• Oreki from Hyouka

P.S. Following the logic that "a character is only as smart as their author", Akasaka Aka is a genius. He was able to show, not tell, the genius of characters in two stories.

Hammer

Yang Wen-li from Legend of Galactic Heroes.

Even though he's a genius, the stupid politics, and backstabbing of his side stop him from winning.