My ten follow.
#1] Lord of the Rings by Tolkien; 2] Conan by R.E. Howard; 3] Tarzan by Edgar Rice Burroughs; 4] The Witch World titles by Andre Norton; 5] The Pooh books by Milne; 6] Narnia Series by C.S. Lewis; 7] The Oz Books by Baum; 8] The first Three 'Ged' Books by LeGuin; 9] Harry Potter by Rowling; 10] Dragon Riders of Pern by Anne McCaffery;
Yes some of these are for kids, but I feel they are the best realized stories and the longest lasting in impact. Each of these writers though not necessarily the first writers of their particular niche in the fantasy market spawned a whole universe of imitators; many good, most so so.