I.... what? What kind of question is this? How does this differ from "favorite book altogether"? Is the assumption that you wouldn't own your favorite book of all time? I mean, that is possible, but also possibly unlikely???
Whatever, sorry to keep harping on these questions, guys, it is obviously something of a fixation for me for some reason.
Okay, going with a strictly literal definition...
House of Leaves, by Mark Z. Danielewski
Hah! Here it is again, just after I slipped it somewhere it didn't quite fit. Welp. If we take this question to mean "favorite book to own," there can be no other. This is a book that deserves being looked at again and again, studied just for the fun of it, reread because there is so much going on, or just opened to a random page and shown to incredulous friends. I love owning this book. So yes, of the books I own, this one has come through for me most often. I think that's an acceptable answer to a slightly redundant question, yes?