It's a Wonderful Life (1946), A Christmas Story (1983), White Christmas (1954), Miracle on 34th Street (1947), etc. From quotes I've seen on facebook, Elf (2003) seems to be an up and coming contender too.
But I'm wondering about some of your favorites that might be less famous. . .
I enjoy the Christmas scenes in You've Got Mail and in While Your Were Sleeping. I also prefer the lesser-known Holiday Inn (1942) to it's successor, White Christmas. Another obscure film with a Christmas scene is Four Jills in a Jeep (1944) about WWII entertainers. Robert Mitchum is so romantic in Holiday Affair. (1949) Finally, The Nativity Story (2006) is a new favorite of ours.
What are some suggestions you have for films that put you in a holiday mood?