It's definitely "It's A Wonderful Life". It's an oldie but the story is wonderful. It's just not Christmas for me unless I get to watch it. Yes, it is 3 hours long and was made in 1946! Talk about a classic!
I haven't gone one Christmas without watching this movie. As a kid it fascinated me and as a young adult I appreciate it even more. It's (in my opinion) one of the most underrated Christmas movies and I'll continue to watch it every Christmas