Personal preference plays a big role. If you like stories with a lot of suspects and a closed - off setting, 'And Then There Were None' might be the best for you. It has a great sense of isolation and a constantly building sense of dread.
Another way is to look at the complexity of the plot and the characters. In 'The Murder of Roger Ackroyd', the plot is full of twists and turns, and the characters are well - developed. The way Christie manipulates the reader's perception of the events and the characters makes it a very strong candidate for her best work.