When selecting the best study materials for UPSC preparation, aspirants should focus on a few core features. First, the materials must be comprehensive, covering the entire UPSC syllabus in a clear and concise manner. Second, they should be well-structured to facilitate effective learning, emphasizing important topics and concepts. Third, consider sources that provide current affairs updates, as these are crucial for both the Prelims and Mains. Lastly, aspirants should look for materials that include practice questions and mock tests to evaluate their understanding and readiness.