Which is Agatha Christie's best book?

My favorite id And Then There is None. I had to read it in 8th grade, and I fell in love with it. I read it again during the summer going into my sopre year in high school. I like it so much because it is a true murder/mystery book. Throughout the whole book, you are constantly trying to figure out who the murderer is. And then just when you think you know who it is, they are murdered. And then the ending just takes you by surprise! It is so good! I think I might have to go get it from the library and read it again now! Lol.