That's hard and - like favorite flowers - for me depends on the season or phase of life.
As a student, I loved Stellar Interiors decades ago and am loving learning to play guitar now for similar and yet very different reasons. Oh, the challenges of relaxing and stretching oneself into completely new ways of being.
As a teacher, I always enjoyed introductory astronomy courses for non-science majors. It was so rewarding to see people understand concepts they'd imagined were too technical for them to grasp.