Here is my list(drum roll please):
5. The Arabian Nights Murder by John Dickson Carr - I hate to start the list with the grand master himself, but JDC has been known for giving some really boring and annoying duds. 'And so to murder' is another book which was I remember being equally sub standard. What pinched it for Nights was the farcical plot filled with absurdities.
4. A Taste for death by P.D. James - Very long and boring. Not a proper mystery, can be used as a cure for insomnia. read the detailed review by clicking on the book link.
3. Trial and Error - The first book I read from Anthony Berkeley was the case of the poisoned chocolates, which was one of the most brilliant mystery books that I have ever read. After finishing poisoned chocolates, I had to get my hands on another book from Berkeley. I ended up with Trial and Error, which I disliked so much that I never got back to reading another Berkeley - The plot is interesting in the beginning but then falls so much in quality that its hard for the book to ever recover. Its also deeply misogynist and insensible.
2. Parker Pyne Investigates by Agatha Christie - Queen of crime she has given so many gems which we will cherish over and over again, but she has also given us so many duds which we will revile every time we remember those horrible books. Parker Pyne is of course the paramount of worst Christie books, it just edged out some other very strong contenders like the 'Nemesis'.
1. Wisdom of Father Brown by GK Chesterton - Worst book series of detective fiction, excruciatingly slow and boring. Bigoted, slow, racist and plain pathetic with very few positives. The whole Father Brown series is worst series in detective fiction for me.