----- Not an ESL teacher -----
But in some contexts there is a difference between them:
#1 Someone 'outside the method,' by means of some 'tools', has proved that it gives the most reliable results. The proof did not rely on the method itself, but rather on other 'external' ones.
#2 The usage of the method itself has proved to give the most reliable results.
You have used the method many times and, by inspection and by direct comparison with other methods it is clear that it produced the most reliable results.
PS1: I assumed in #2 you also meant the most reliable.
PS2: This is just an interpretation, it sure will depend on the context. For example it is not strictly necessary that in #2 the method was used many times.
Student or Learner