Jing-mei is a complex character, and there are several traits that could be singled as her best. Her empathy and connection to her mother are some of her best qualities, but these are in part due to her other traits, including her stubbornness, intelligence, independent spirit, and self-awareness. Her mother tells her that there are only two kinds of children: obedient ones and independent ones. Jing-mei's struggle is that she is independent but tries to be obedient. This is a bitter experience for her, and she acts out in various ways, such as purposely not practicing the piano and mortifying her mother at her first recital.
But it is also clear that Jing-mei understands that there is more to her mother's behavior than a simple desire to maintain discipline. The episode at the New Year's Eve dinner, when she and her mother take the worst crabs, is indicative of her mother's understanding of Jing-mei's character, and the moment when Jing-mei recognizes in her mother the same self-sacrifice and fierce independence she recognizes in herself. Like her mother, Jing-mei is after something more complex and authentic than embodying a single "best quality." In that sense, her best character trait can be considered her willingness to look further than most and to see people for who they really are.