Age Before Beauty

Question: Which of the K'uh would be considered the ruler of the pantheon, of the likes of Zeus, Odin, Shango, Amaterasu, etc.?

Sorry, but the Maya are having none of your European imperialist pantheon structure today! There's no official king or queen among the K'uh; the individual gods function as the lords of their particular areas of influence without answering to any higher authority. There are few political conflicts in Maya myth (other than the spectacular clash between the gods of life and the gods of death in the Popol Vuh), and most of those have to do with kinship and familial ties rather than rank or title. That's not because the Maya themselves didn't have a concept of rulership - quite on the contrary, they controlled a massive empire, had extremely complex systems of lineage and aristocracy and held their kings in such high esteem they believed them to have a direct link to the gods - but more because they didn't conceive of the gods as necessarily behaving like humanity in that way. Their job is to be out there being in charge of stuff like the sun or the ocean or the jungle; the K'uh are pragmatic gods who all have particular natural functions, and being in charge of the other gods really isn't necessary under those terms.

If you're set on having some figureheads to represent the K'uh in an appropriately regal fashion at official meetings of divine leaders, however, we'd suggest using Itzamna and/or Ix Chel as the de facto "leaders" of the pantheon. As the ancient creator gods of the pantheon, they're among the oldest deities of the K'uh and wield enormous elemental power, and are literally the ancestors of some of the other gods (most notably their son Hun Nal and grandchildren Hunahpu and Xbalanque, but possibly others, too). They occasionally appear doing things that suggest that they're at least respected and deferred to thanks to their age and status as creators of the world, so if you want someone to be the authority in an internal pantheon dispute or the personages to be addressed by visitors, it's probably them.

To step beyond the bounds of our supplement for a moment, you also have the option of using Hunab Ku (literally "sole god"), whose name you will probably run across in reading about the Maya gods. Hunab Ku appears relatively late in Maya literature, after the arrival of the Europeans, and is commonly referred to as the greatest of Maya gods and/or the creator of the world and all other things in it. He's a controversial figure, however, because scholars are not sure he actually ever existed; the phrase was used heavily by the incoming Spanish to refer to the Christian god they were trying to convert the natives to worship of, and as a result the later references to him in the writings of Maya and Christian historians of the time are suspect. It's possible that he's totally a legit and original Maya god, an ancient, formless creator, but at the same time it's also possible that the Maya adopted the phrase and invented him as a member of their pantheon in response to the influence of Christianity, or that the Christians involved simply decided that the Maya were secretly monotheists who just didn't realize it and therefore wrote a lot about an all-powerful father-god they didn't actually have (which happened in various cultures more often than you might think).

If you do want to treat Hunab Ku as a legitimate member of the K'uh, he'd be without a doubt the top dog of the pantheon, but he's also so almighty and distant that he may be relegated to the land of gods who are too large to be played along with folks like Olodumare and Ahura Mazda. We lean toward assuming that the mentions of Hunab Ku are really talking about suspiciously similar creator god Itzamna, renamed by Christianized writers, so if you'd like to borrow the super-powerful imagery of Hunab Ku for him, he becomes the natural choice for pantheon leader.

Considering the K'uh propensity for swapping personas whenever they feel like it, it might actually be a lot of Storytelling fun for them to have a rotating "leadership" based on who is what today. If Itzamna's off busy being a young female lizard somewhere for some inscrutable reason, rather than his normal old male self, someone else more appropriate might have to step in to drive for a while, leading to other pantheons finding themselves frustrated and confused about why they never seem to be talking to the same monarch twice.