The head of state or ruler was the highest-ranking member of a government. Emperors, kings or queens, and presidents usually served as head of state of their respective polities.
In 2364, William T. Riker defended his choice to adorn himself in the native attire for males on Angel I because of diplomacy. According to Riker, "I have requested an audience with a head of state, and I will honor her by wearing indigenous apparel." (TNG: "Angel One")
Salia of Daled IV had the rank of head of state by 2365. (TNG: "The Dauphin")
Acost Jared was the Ventaxian head of state in 2367. (TNG: "Devil's Due")
In 2373, Kira Nerys requested that Tekeny Ghemor be treated like a visiting head of state during his stay on Deep Space 9. This included being given full communications and computer access, and any assistance he may need from the station's crew. (DS9: "Ties of Blood and Water")
The Chancellor of the Klingon High Council was the head of the Klingon Empire, even after a new emperor was installed in 2369. (TNG: "Rightful Heir";DS9: "Apocalypse Rising")
In 2373, Gul Dukat became the new head of the Cardassian government. (DS9: "By Inferno's Light") Later, Legate Damar was the head of the Cardassian Union. (DS9: "Strange Bedfellows")
Grand Nagus Zek was the head of the Ferengi Alliance. (DS9: "The Emperor's New Cloak")