My favorite will probably be The Executor's End, by Heimlich VonVictor - that story was so brillianty formed and very interesting in it's own way - I remember when I first played it, I had to quit it at least twice, due to clam ups and not getting the required questions done to lead to the eventual whodunnit - person :(
Furthermore, I always liked Joseph Zeo's mysteries. A nice plot, lots of interesting and developed characters... It has it all :)
A special recommendation I want to give to "Wheel of Misfortune". That one was very interesting, because from what I recall, you really had to pay close attention to what everyone said, otherwise, it could end the story very badly.. and that's why I love it :) - because it forces you to pay attention to even the littlest details and ask: "Is it worth remembering or not?"