What is the single most important thing in Sales & Why ?

The success of sales cannot be defined to only one "thing" or KPI, it all depends on the type of sales you are in. 

My philosophy: The right product at the right price combined with outstanding customer service = success. If you cannot get that right, it will be a challenging task.

Note: It all depends on which sales trade you are in, other trades requires more than just the above mentioned.

Rapport: It is commonality of perspective: being “in sync” with, or being “on the same wavelength” as the person with whom you are talking.