There's really no contest here. Jane Austen is the only author whose books haven't disappointed me. I've loved every single one of her books. The only other authors who have achieved this have only written one book, like Margaret Mitchell. Other authors like Tamora Pierce, E. Lockhart, and George R. R. Martin who have written some of my most beloved books have also written books that didn't work for me. For this reason, Jane Austen is my favorite author.
Honorable Mentions: Margaret Mitchell, E. Lockhart, Melina Marchetta, George R. R. Martin, and Tamora Pierce because they're still great writers.