The scene where Clark gets locked in the attic. It might not sound that heartwarming, but while he's up there, he finds old recordings of his Christmases as a child from the 1950's. It's not that hard to notice a few Manly Tears running down the side of his face.
Clark watching the home movies is the first insight into why Clark is doing what he does. He doesn't just want to have a fun old fashioned family Christmas, he wants his fun old fashioned family Christmas. The magic of those old Christmases is why Clark insisted on having the entire family over for Christmas and it gives Clark's fanaticism real emotional depth.
Also, his final line. Despite the hellish disasters all December long, despite the near loss of life and sanity, in the end, he gets what he really wanted: A loving family experience at Christmas.
A small one at the end when the credits begin to roll, Clark, with a sense of accomplishment from the above entry still lingering, pets Cousin Eddie's dog Snots, the one who everyone is afraid to touch due to his sinus issues.
The scene where Clark is reading "The Night Before Christmas" to the family. It's one of the quieter moments in the film yet it warms your heart at seeing everyone together and enjoying themselves.