Depends on what you want.
For the genuine medieval tales in English translation or modernization see:
http://www.amazon.com/Medieval-Arthurian-Modern-English-Translations-%231/lm/R3KPSDORBZLMJ6/ref=cm_lm_byauthor_title_full
http://www.amazon.com/Medieval-Arthurian-Modern-English-Translations-%232/lm/R2S7V81SU470N6/ref=cm_lm_byauthor_title_full
http://www.amazon.com/Medieval-Lancelot-Modern-English-Translations/lm/R2SR3FRVTQ2CIU/ref=cm_lm_byauthor_title_full
http://www.amazon.com/Medieval-Tristan-Modern-English-Translations/lm/R3JLROH1RM9MI0/ref=cm_lm_byauthor_title_full
For modern retellings and novelizations try:
“Idylls of the King” by Alfred Tennyson. This is available at http://ebooks.adelaide.edu.au/t/tennyson/alfred/idylls/ and other websites. It had an impact on 19th century readers similar to the impact of Tolkien int the 20th century.
“The Story of King Arthur and his Knights', “The Story of the Champions of the Round Table”, “The Story of Sir Lancelot and his Companions”, and “The Story of the Grail and the Passing of Arthur”. Though written in Malorian style, Pyle often changes (and bowldlerizes) his sources.
“The Sword in the Stone' and “The Once and Future King” by T. H. White. “The Sword in the Stone” appears again as the first part of “The Once and Future King”, but in a somewhat different version as paper shortages prevented White from including “The Book of Merlyn” in the original publication of “The Once and Future King” so he replaced some of Arthur's adventures in the version of “The Sword in the Stone” which appears in this book with some material from that story. As a result, Arthur's adventures with the ants and geese now appears twice. It is best to read “The Sword in the Stone” by itself, and then jump to “The Candle in the Wind” portion of “The Once and Future King”. Based almost entirely on Sir Thomas Malory’s “Le Morte d’Arthur” when it it not White's own invention which much of it is.
“The Mists of Avalon” by Marion Zimmer Bradley. This is a feminist reworking of Arthurian legends, and owes much to Bradley's own imagination.
“The Crystal Cave”, “The Last Enchantment”, “The Hollow Hills”, “The Last Enchantment”, (“The Prince and the Pilgrim”), “The Wicked Day” by Mary Stewart. “The Prince and the Pilgrim” is a separate tale, written later, but set in the same world.
“Arthur Rex: A Legendary Novel” by Thomas Berger. A somewhat comic reworking of Arthurian tales.
“The Winter King A Novel of Arthur (The Warlord Chronicles)", “Enemy of God: A Novel of Arthur (The Warlord Chronicles)”, “Excalibur: A Novel of Arthur (The Warlord Chronicles) by Bernard Cornwell.
There are many other modern works, but the above, I believe, are the most popular. But do not confuse what they say with the original medieval tales.
See also http://www.lib.rochester.edu/camelot/arthmenu.htm for a guide to many works available for free on the web.