Grandma has many classics, which one is the best, and it has always been difficult to draw a conclusion. Personally, no one is alive is the best, but it is a bit atypical. Strictly speaking, it is not a detective novel (spoiler). There is also a very subversive "Roger's Mystery", which is regarded as a classic by many people. I felt very uncomfortable after reading it, and thought it was a trick of my grandmother.
The representative work of grandma's style, which is accepted by everyone, is Murder on the Orient Express.