Day 23 was 75% less exciting than Day 22, with Poul Anderson easily whipping Fredric Brown. Anderson will go up against Leigh Brackett in round 3.
Day 24 is Michael Moorcock versus L. Sprague de Camp.
If only the Hawkmoon and Corum and Jerry Cornelius novels existed to represent Michael Moorcock's fantasy output, he would be well regarded. The Eternal Champion series provides a rich tapestry of ideas, from the thoroughly 60s/70s oddity of Cornelius to the post-apocalyptic Hawkmoon to the rich Celtic overtones of Corum. But, of course, he launched his career with the most enduring of his characters, Elric of Melniboné.
For D&D purposes, the most important thing Moorcock did was to take the Law and Chaos conflict in Poul Anderson's Three Hearts and Three Lions and run with it. D&D proceeded to do likewise, and alignment has shaped its universe in subtle and not-so-subtle ways. Elric's runesword of course is copied in the classic module White Plume Mountain, although D&D has never been good at the kind of summoning magic he performs.
The Elric novels are mostly fix-ups, and recent re-releases have included the earlier forms. Moorcock is the sort of author who always has room to tinker with his past creations, and filled out a surprisingly long backstory for his albino sorcerer. But in terms of sword & sorcery anti-heroes, he remains one of the most compelling, his glooms the deepest, his savage bouts with Stormbringer among the best action.
L. Sprague de Camp
If you judged merely by what he did before 1966, L. Sprague de Camp would clearly rate as a significant fantasy author. His collaborations with Fletcher Pratt are among his finest work: the Harold Shea works, Land of Unreason, The Carnelian Cube, and so on. And his anthologies such as Sword and Sorcery and The Spell of Seven were important in defining swords & sorcery as a genre.
But de Camp also edited the 1960s Conan paperbacks. This would seem to be a good thing - after all, they popularized the tales of the Cimmerian and brought Frank Frazetta's iconic art to the character. Had these books not also included original stories by de Camp and his protégé Lin Carter, and had they not rewritten unfinished Howard stories, de Camp's reputation might have been sterling. Instead he is heavily disliked by Howardian purists.
de Camp's work was credited by Gygax as a major inspiration, even if his rationalist streak seems a bit too skeptical for D&D. His Viagens Interplanetarias series was used for GURPS Planet Krishna, and his Harold Shea books are a solid go-to if you want to incorporate elements of myth and classic fantastic literature. He was, deservedly, a Grand Master in his lifetime. But one must decide where they stand on his editing work.
You can vote in the poll here.