.... Well, I read a lot of sad books, so I'm not sure which one is the saddest, but here is my personal opinion on the ones you listed, and a few of my own.
TFiOS: Well, I personally hate this book for a few reasons. The first is that it hits waaaaaaay to close to home for me. (No, I do not have/had cancer, and no, Neather did my boyfriend) also, the whole thing is practically just saying there is no reason to live, which isn't exactly to comforting for a person like me. *coughs* So there isn't much room for sadness in it for me.
The Book their: this one was a stellar book, and the movie was almost just as good.(which is rare) The end was so, I don't know, not beautiful, but something like that. When I finished it, I seriously think I just sat there and thought for about ten minutes.
DoD: Concifering this wasnt a good book, I kind of shock myself when I say it is up there with some of the sadder books. (Not nearly the saddest books though)
The Titantic Book Three(I cant think of the title eather): Again, it's sad, and it's a great trilogy, but not one of the saddest.
Ok, here are a few of my own;
After Iris: this one is usually not concideted a sad book, but I thought it was really touching at some points. It's one of my favorite books, and again, hits close to home, but in a good way. :')
If Only: this is my all time favorite book, and is so bittersweet. It is absolutely a Beutiful books, and has a lot of emotion in it. (I know that sounds really corny, but it's soo true) there are some parts in it, like the moment on the soccer field, where your you want to cry right along with Coni.
13 reasons why: this one definitely fits the qualifications of "sad". Just a heads up, this one is really mature, dark, and heavy. It is really sad, and it is the nature of the book.
Counting By 7ns: this one is really bittersweet as well. Even though there are joyful parts, there are also sad ones.