In some iterations of Final Fantasy, Bahamut is King of Dragons because he usurped Tiamat, so it's possible Tiamat would be the Primal of Dragons. I suppose Bahamut is the best option, by far, but it also seems... I don't know. I don't like the idea of the dragons having the Primal so devastating he caused at least two umbral eras and was imprisoned in a moon as theirs. You'd also think the summoning of Bahamut would be something Dragons venerated, but they didn't get involved at all. I don't know who else it could be, but if the Dragons still venerate Bahamut they have a funny way of showing it.