The letter from someone who taught you is OK for PhD application. For application of faculty position,you need to have much more interaction with the reference, especially in research.If 某大牛 teaches and also has a research project with you, as long as you are great at research (for instance, publishing a good paper with him/her), I don't think the grade of B or even C will ruin the letter. Hu cares about the grade of a PhD....