Must. Stay. Awake.
On the WaPo Celebritology Blog, there is an interesting thread about movies you can watch again and again and again and again ...
Here is my list (alphabetized, so I don't play favorites):
Crouching Tiger, Hidden Dragon
A Fish Called Wanda
Harry Potter 1-4
Lord of the Rings Trilogy
Singin' in the Rain
There are others that I *could* watch again and again, but I don't own them (yet).
Feel free to leave your choices in the comments.