Favorites are hard, but if i had to chose a movie, my choice would have to be The Royal Tenenbaums.
Wes Anderson always seems to present the complexities of being human from an often hilariously stark perspective. From a purely artistic point of view his compositions are very well done, and show a good sense of balance and color choice. Each shot has so much character and truly presents his version of the story.
Ive turned to his movies in the past to help me prepare for designing and arranging things in my portfolio, for the feelings of clarity and subtlety in each shot.