I believe it must be Tom Bombadil? So old that even the Ring has no power on him? Sadly we read so little of him though.
"Eldest, that's what I am... Tom remembers the first raindrop and the first acorn... He knew the dark under the stars when it was fearless - before the Dark Lord came from Outside." —Tom Bombadil (The Lord of the Rings)
And he is older than Sauron
Tom Bombadil was in the world when the Elves passed westward before the seas were bent. He knew the dark under the stars when it was fearless – before the Dark Lord came from Outside.
Even Elrong agrees that he is the oldest
"But I had forgotten Bombadil, if indeed this is still the same that walked the woods and hills long ago, and even then was older than the old." —Elrond, The Fellowship of the Ring
You can check these books: "The Letters of J. R. R. Tolkien" and "The Adventures of Tom Bombadil" or feeling lazy? Check his wiki Also read theoris about him