I recently read a book whose protagonist was what I might call the ultimate consultant.
Her role was essentially to perform a single act, which was to say Yes or No.
Essentially, she would look at a design of something like a logo, and decide in a split second whether or not it would work, according to her intuitive understanding or marketplace trends. Written into her contract was the fact that she would give no explanations, no advice and no further comment on the matter.
For this she was flown around the world, and paid very well, which was testimony to her instinctive and unique ability.
She is the ultimate consultant in my eyes, providing maximum value in the few seconds it took her to do what she did. At the same time, the many hours that she spent studying fashion trends around the world were done at her own expense, which hopefully is something that her clients understood!