In the context of the Marvel Cinematic Universe (MCU), "king" can refer to a few different characters, but the most prominent figure often associated with kingship is T'Challa, also known as Black Panther. He is the king of Wakanda, a fictional, technologically advanced African nation. The character was portrayed by Chadwick Boseman until his untimely passing in 2020.
Additionally, in the series, we also have King Thor and King Namor (from "Black Panther: Wakanda Forever"), who hold significant status in their respective narratives.
If you meant the overall leadership or creative direction of the MCU, that would generally point to Kevin Feige, the president of Marvel Studios, who has been instrumental in the success of the franchise.