King Arthur - Recommendations

I'm currently reading "The Mists of Avalon" by Marion Zimmer Bradley, and while I really like it, I feel like I am missing out by not starting at the beginning of, well, Arthurian literature? And I come here and read about Tennyson's "Idylls of a King" and I had never even heard of it, I though the Lady of Shallot was just a standalone, shame on me. So now I'm looking for recommendations on the basics of the legend and its derivations over the years. Of course I googled, and did some bibliography myself, but do I start with Malory, or Monmouth, or is there something before that that I need to read (because it's beautiful, for example)?
So everything from The Beginnings to modern adaptions (I only know about the "Mists" and T.H. White, to be honest) would be really, really welcome, and thank you in advance!