Geez, these are getting harder and harder. I love mysteries, and I love a great many series, each in their own way. It looks like Sherlock Holmes has already been given his due on my feed, so I'm going to go with Agatha Christie. But now I have to pick my favourite Agatha Christie.
This is just too hard.
Honourable mention goes to the marvellous The Murder of Roger Ackroyd, and to Death on the Nile, which contains the "Do not open your heart to evil" line that has been so excellently parodied in yet another of my favourite mysteries.
I think Labours of Hercules wins the prize because I absolutely adore its grand conceit: Hercules Poirot plans to retire, but before he does, he decided to take twelve cases that must be analogous to the original Greek guy's labours. What follows is a lighthearted and amusing collection of loosely-connected short stories that are guaranteed to induce a chuckle or two.
The Nemean Lion is a Pekingese. Just sayin'.