It's so hard to choose favorite lines and scenes! From the movie, my favorite scene is in the beginning when they are getting ready for the job. Mal just told Jayne no grenades. The conversation between Jayne and Zoe in the background always makes me giggle and Jayne's little awwwuh at the end.
For the series...again, would have to be Jayne. Our Mrs. Reynolds. When Jayne shows up holding Vera, cocking the gun and Mal thinks Jayne is going to shoot him. The expression on Jayne's face, the reverence and love he has for Vera.