I think it's an impossible question to answer.
+1 on this. First of all, think of the four main branches of the mystery genre tree:
*the cozy mystery
* the "hard boiled" private eye mystery
*the police procedural
*the spy/international thriller
Each have their different pleasures, advantages, and drawbacks- pleasures that that often contradict with each other. With the "cozy mystery", for example, you are shielded from the reality that there is a violent crime and a corpse at the center of the mystery , whereas the police procedural practically rubs your nose in it.
Fans of one variety are unlikely to consider examples of another variety as " best or even "good". Think Val McDermid , with her graphic descriptions of violence and unabashed talk of sex vs Dorothy Sayers: we are practically talking of different planets!!
Maybe we can talk of a best subgenre of mystery fiction: best police procedural, etc.
That at least is conceivable.