If you have read a lot of fantasy, there's a good chance that your favorite is a book that was popular at some time. My favorite, for example, is John Crowley's Little, Big. It's not obscure! It won critical acclaim and wide readership in 1981.
Most popular books become obscure books, given enough time. (Reading Les Miserables, I had to consult the footnotes repeatedly for Victor Hugo's allusions to then-popular novels and novelists, almost none of which remain popular today.) If you're a serious reader the chances are diminished that your favorite book is something that has just recently been written. But it's also likelier than random chance that your favorite book was once popular, because as you note there's generally a positive correlation between quality and popularity.
Most popular books become obscure books, given enough time. (Reading Les Miserables, I had to consult the footnotes repeatedly for Victor Hugo's allusions to then-popular novels and novelists, almost none of which remain popular today.) If you're a serious reader the chances are diminished that your favorite book is something that has just recently been written. But it's also likelier than random chance that your favorite book was once popular, because as you note there's generally a positive correlation between quality and popularity.