I don't read much mystery (if you will recall, I've never read Agatha Christie). Recently, my book club selected an Anne Perry book, The Face of a Stranger, featuring peeler detective William Monk. I like him just fine, but the character I really liked was his occasional assistant Hester Latterly. She's not technically a detective, but she plays such a pivotal role in solving his cases, I'm going to choose her anyway.
Here is my original review of the book: Face of a Stranger.