The one that is remembered long after most of the gifts are forgotten?
I'd like to think my DCs will remember some of the toys they've got, and I think they will have fond memories of sharing the books they've been given (even though books never really get a look in on the day itself!)
But I think the one thing she will always remember is coming downstairs last year to see her favourite painting (starry night - she got obsessed with it when they did an art topic at school) on the wall - she'd not asked for it, but we'd got a poster of it and a nice frame, and Father Christmas hung it overnight. Her face was the best thing ever! She still looks at it happily all the time and talks excitedly about when Santa gave her the painting. I really think (hope) that moment will stay with her. It will stay with me, anyway. It's not even a painting I love that much but every time I look at it it makes me happy because of how happy she was to see it :)