Who's the oldest?

Alright. Let's get some things down.
nullRealize that these conclusions are based only on what I've read in these posts.
Tom Bombadil, although decided unimportant to this topic, is described by burrahobbit as the song which is Arda. Therefore, rather than the oldest on Arda, he is Arda.

Balrogs are corrupted Maia.

Treebeard, while not necessarily the oldest being, is the oldest Firstborn still around ME.

Ok. Question, though, did anyone answer the mystery of Shelob? I have not read HoME, are there answers there?

Thanks for your time...