Who do you think Tom Bombadil really was

[Ibrîniðilpathânezel]

I would be disinclined to think that Bombadil was in any way Eru, since it is plainly said that he would fall in the end before the power of Sauron and the Ring, "last as he was first." In letter 181, Tolkien said, "There is no 'embodiment' of the Creator anywhere in this story or mythology." I would say that pretty well eliminates the Bombadil as Eru concept (sorry, alatar). But it would not eliminate Bombadil "the enigma" as a powerful agent of Eru. I would be more inclined to think that if Bombadil is not a Maia, he is a Vala who, like Tulkas, came after the Ainur first entered Ea, and took up residence on Arda when it was finally formed, perhaps without the knowledge of the other Valar. He may have been planted in Middle-earth by Eru as a potential ace-in-the-hole, so to speak. Well, it's a thought.