What is a good grade for an MS student expecting to apply for a PhD in a top-tier school?
Anything above a 3.5. Grades higher than that are unlikely to make much of a difference for your application. The most important parts of your application are letters of recommendation, research experience, and personal statement. As long as the GPA is above 3.5, with no unexplained low grades in highly relevant courses, the precise GPA is not relevant.