Rather generic advise here but it's probably applicable if your professor really asked you this; the rec letter's that make a difference difference do two really basic things a) they are specific b) they find a way of putting in perspective your strengths; that is "ranking" them based on what your professor has seen their entire carer. It's not just Student X has the third highest grade. There are many other things a professor can cite that make you unique and really stand out to them in their entire career. If you emphasize the importance of that in a rec letter, and not generic platitudes, things that can be looked up in other parts of your applicationor a description of the class as opposed to a description of you(a rather common thing amongst many rather lukewarm letters) it will help you immensely.
