The Big Bang Theory had five Christmas episodes, each of which has classic moments. I think my favourite is probably the first of them: "The Bath Item Gift Hypothesis" (Season 2, Episode 11), in which Sheldon attempts to buy gifts according to a reciprocity principle that demands each person swap presents of equal value. Second is maybe "The Cooper Extraction" (Season 7, Episode 11), in which Sheldon is back in Texas because his sister is having a baby; and, in his absence, each of the others imagines how their lives would have gone if they'd never met him.