The most helpful grade I ever received on an essay was an 'F'.

It was in my first year of university. I knew the course and knew the material but couldn't have been arsed to put together a paper which adequately addressed the subject.

The comments the professor wrote were scathing and it was a wonderful full slap on the face that I'd better get my act together and actually deliver what was expected of me if I ever wanted to accomplish something of value.

I did. Thanks for that 'F'.