I agree with Michigancat on Christmas Vaca and Home Alone for the most part, but I still think Home Alone is number two.
Christmas Vacation is number one and it isn't close. Cousin Eddie alone nearly puts it at the top. I like the subtle goofiness of the late 80s humor in the movie and I find something else that is funny every time I watch it. I love the difference in Clark's interaction with his Dad and with his father-in-law.
Home Alone is number two for me, a great mix of nostalgia and childhood humor. I don't really find it that funny any more, but it's sweet. I probably like the scenes with the old man, and in the second one the old lady the most as an adult.
Elf is funnier than people give it credit for. I laugh a lot when I watch that one.