when? WHERE? whats his name? His blood type?
lol, can't recall a king of hyrule in OOT or Wind Waker or LTTP.
There was a king in A Link to the Past. He appears in the game's intro sequence that shows when you leave the title screen running, and he appears in the game's ending alongside Zelda. He's just an idiot who was tricked by Aghanim and presumably stuffed in a closet or killed until the Triforce fixed everything.
There is definitely a king in Ocarina of Time. He is just off-screen to the right during the scene when Link and Zelda are spying on Ganondorf through the window. Who else would Ganondorf be kneeling in front of?
So in A Link to the Past, Ocarina of Time, Minish Cap, and in the backstory for both The Adventure of Link and Wind Waker, Zelda is a princess who is the daughter of a living king who is still on the throne.
Now, in Skyward Sword and during the events of Wind Waker/Phantom Hourglass, there is a Zelda, but no actual Hyrule, so she isn't a princess at all.
That still leaves a few versions of Zelda for whom it isn't clear if she is the ruler or not. We know pretty much nothing about the Zelda from the first game. Most non-canonical manga and comics give her a father who is still in charge. The sleeping Princess Zelda from The Adventure of Link, as I mentioned above, definitely wasn't a ruler, and she probably isn't going to be queen after waking up without a coup d'etat given that there is another Princess Zelda alive at the same time who has a better claim to the throne. I think the both A Link Between Worlds and Spirit Tracks Zeldas are pretty much the rulers, but both seem to lean heavily on advisors (like Impa or Chancellor Cole) due to their young age. They might be considered regencies, and the princesses will probably be officially crowned when they become adults.
Maybe there is just a tradition in Hyrule for reigning Zeldas (since they are all named Zelda by law/tradition) to use Princess as their official ruling title? As people mentioned above, there is nothing inherently wrong with using Princess as a ruling title, it is just strange being paired with male rulers who use King as their title. Of course, it is a fantasy kingdom that uses a different writing system that we are presumably seeing translations for.
Maybe the system just changes during different eras? It is a pretty long timeline with some pretty big events shaking the kingdom. Sometimes you wonder if there is even any dynastic continuity at all.