By smoking gun, I mean THE thing that cinches the case and your theory together.
It could be something as “small” as, say, the porch light (in the case of the Springfield Three). Or it could be something big – outside DNA, Mr. Bojangles (in the WM3 case), etc. It could be someone’s behavior or someone’s alibi.
Your smoking gun could even be “X happened, which makes me believe that John Doe is totally innocent of the crime
Another way to think of this question: what one item – piece of evidence or whatever – holds the most weight with your theory? What tips the scales for you?
Links are definitely encouraged, especially for the lesser known cases!