The best mystery story that I ever read would probably be  "The Oblong Crate" . The author kept making guesses of what was in the crate.  But all those guesses were wrong. It wasn't  until the captain finally revealed what was in the crate that I stopped guessing. For a moment I thought they wouldn't announce it. That is what a good mystery needs. He wanted us to predict and get those kinds of conclusions so that when we finally think we know the answer he changes it up and goes the other way like a U turn.  I love mysteries that is my favorite genere (even if it's scary).  So this is why  "The Oblong Crate" was the best mystery I ever read.

Story starter:

"GET OUT !" The old man screamed. He chased the kid with his cane shouting " YEAH YOU BETTER RUN AND DON'T COME BACK AGAIN !" Jack almost having a heartattack said " Whew, Sarah that was a close one." Sarah replied " See Jack I told you not to go inside his house."  Jack and Sarah were two adventurous kids who always got into trouble. Jack said " Sarah I really wonder what was in that red box he had." 

" I dont know Jack we better go before.... AHHHHH=http://