First you should clarify us what or who is the best digital marketer?
It is very broad, slippery and dynamic field at the same time. However, no precise criteria exist onto how to "asses/evaluate" marketers. There are bunch of metrics available, of course, but it is becoming increasingly challenging to track the concept that you are looking after.
For a start, it's the best to start from the stretch. Please have a look onto attached material.