Tom Bombadil, but it seems unlikely that he would be Iluvatar. I like to think that he is Tolkien himself, and Goldberry is his wife.
If Tolkien, we're talking a whole nother kind of creator, are we not?
That’s a real good point about Tolkien. When someone why he asked why a man of his stature (he was one of the best linguist of the twentieth century) wasted time with all of these “fairy stories”, he responded, “Because I was made in the image of God”
He truly believed that God created us in his own image, and that meant that mankind had all the aspects of God built into him. One of those aspect is “Creator”. So Tolkien viewed fiction, world building, invention, myth making, fairy stories, etc. as expressions of the “Creator” in all of us. Man was driven to create, because the Creator made us in his own image.