That question is a lot like the question about the favourite fantasy novel. How do you expect me to choose one?
Well-Schooled in Murder: I am falling out of love with Elisabeth George...or rather I already have because I didn't even manage to finish her most recent one but that does not change the fact that her early work is awesome. There used to be only 2 1/2 authors that could make me go 'Oh I'll just read a bit more and then go to bed...why is it suddenly 2 o'clock at night?' and they were George, Rowling and occasionally Pratchett
The Fifth Woman: I start with a confession: I have issues with many of Mankell's books and don't think he is as awesome as he is sometimes made out to be. However some of his books aren't just good. They are brilliant and The Fifth Woman is one of them.
The Cuckoo's Calling: Yes I have no trouble admitting that. I think it's great. In fact it reminds me a lot of Elizabeth George's earlier (and better) books.
Murder on the Orient Express: Do you really have to ask?
Strong Poison: *making incoherent squeing noises while yelling 'I LOVE PETER AND HARRIET'*