1. Earlier than Geoffrey of Monmouth: Gildas, Bede, and Nennius
They mentioned the existence of a British war-chief who defeated the Saxons at Badon Hill, but did not call him "Arthur."
2. Geoffrey of Monmouth, a Welsh cleric (c. 1100 - c. 1155 CE).
Geoffrey is the first to introduce characters who will become integral to the later legends: Guinevere, Merlin, Sir Kay, Sir Bedevere, Sir Gawain, Uther Pendragon, and Mordred. In his History of the Kings of Britain(Historia Regum Britanniae in Latin), Geoffrey also adds touches like Arthur's lance and sword having names and being especially powerful, Arthur as a mighty but merciful warrior, Mordred's treachery in seizing the kingdom and holding Guinevere, the cataclysmic battle between Arthur and Mordred, Arthur's fatal wounding and departure for the Isle of Avalon, and Guinevere taking the vows of chastity and joining a holy order.
3. Wace(Robert Wace, c. 1110-1174 CE) of Normandy
Wace translated the work into Old French vernacular but provided much more than a simple translation. Wace's poetry elevated the story as well as adding significant details such as the Round Table, where all the knights of Arthur's court were equal, and a more complete image of the king himself. Wace is also responsible for the famous name of Arthur's sword: he changed the name from Geoffrey's Caliburn to Chaliburn which, when it was translated to English, became Excalibur.
4. Chretien de Troyes(c. 1130 - c. 1190 CE), a Provencal poet
Chretien added the touches of chivalry, courtly love, and one of the most enduring of all Arthurian characters, Sir Lancelot. Lancelot is the central character in a story detailing his frustrating attempts to rescue the Lady Guinevere. This story is also the first mention of their famous affair which would inform much of later Arthurian literature. Chretien began a tale of Perceval involving the quest for the Holy Grail and introducing the character of the Fisher King.
5. Layamon(c. late 12th/early 13th century CE), a priest of Worcestershire
Layamon was the first to translate Arthur's story into English. Layamon's Brut is a poem of a little over 16,000 lines drawing largely on Wace's work but supplemented by others. Layamon introduced the details of the magical birth of Merlin, describes the origin of the Round Table, and provides the more mystical aspects of the legend.
6. Sir Thomas Malory(c. 1410-1471 CE)
Malory compiled, edited, revised, and added to the legend to create the English prose masterpiece Le Morte D'Arthur c. 1469 CE while imprisoned. Malory's work is the quintessential Arthurian legend as it is recognized in the modern day. It was published in 1485 CE by William Caxton as part of his initiative to place worthwhile literature in the hands of readers through the new device of the printing press. The story was so popular that second and third printings were ordered.
7. Edmund Spenser(1552-1599 CE)
Spenser attempted to revive the Arthurian tales in his epic allegorical poem The Faerie Queene (c. 1590 CE) but his Arthur is far too perfect to be interesting.
8. Alfred Tennyson(1809-1892 CE)
Alfred, Lord Tennyson (1809-1892 CE) popularized the legend through his works beginning in 1832 CE with the publication of his poem The Lady of Shallot, continuing with others on the same theme, and most significantly with the 1859 CE publication of Idylls of the King. Tennyson had long been fascinated by the Arthurian legend and the figure of Arthur and reshaped Malory's text, returning the story to poetic form, to reflect the values of Victorian England.
9. Modern writers after Tennyson
Mark Twain expanded an audience for Arthur in his A Connecticut Yankee in King Arthur's Court in 1889 CE. In the 20th century CE, writers like T.S. Eliot(The Waste Land), Hemingway, Fitzgerald, D. H. Lawrence, and Joyce would draw on the legends for symbolism in their own work. John Steinbeck(The Acts of King Arthur and His Noble Knights) rewrote the tales for a modern audience and T.H. White redefined the legend for his own time in 1958 CE through his work The Once and Future King.