I did stipulate favorite as opposed to best. I thought a lot about it and decided I didn't want to have to include brilliant books I would likely never read again or exclude books that gave me enormous pleasure to read but that weren't necessarily fine literature.
Books we love will resonate with us in a way that they won't resonate with others. For instance, The Bear and the Nightingale trilogy takes place in Rus' under the yoke of the Tartars and its a fantasy series about warring Russian spirits. A fantasy series in which Christianity is on its way to becoming dominant, but the Old Russian gods and spirits still live amongst the Russians. I absolutely love the premise because I love the idea of pre-Christian Russia. We come to books with different perspective due to different histories and as such, what is my favorite will not be someone else's favorite. And that is great because my enjoyment of books is not (one notable exception) dependent upon a book being enjoyed by others. Having said all that, when Joe loves a book that I love, my enjoyment of that book is greatly increased as I can discuss that book with my favorite person.
Favorite is a great way to rank books because it make it personal.