I don't measure books against each other (at least, not in a "this is a standard the rest of you aren't achieving" way). There are certain books and genres I reread and enjoy each time, but usually for different things.
"The Daughter of Time" by Josephine Tey, because it reminds me that history isn't real.
"A Surfeit of Lampreys" by Ngaio Marsh, because it is so funny and yet centres around a horrible act.
"Beauty" by Robin McKinley, because it so completely makes me care about the people and their lives.
And many others.