Favorite would imply one. There are many many movies that I love and enjoy watching over and over; however, if I had to choose favorite it would be American Beauty. An odd choice, I know, but when I watch that wacked out film I always walk away feeling better about life, appreciating the beauty in the smallest things, and just all around with a much more positive attitude. Anything that effects me that much, gets my favorite vote.
Other awesome films/series:
Lord of the Rings
Lethal Weapon
Smokey & the Bandit 1&2
Comic films: Captain America, Thor, X-Men, Spider-Man, Batman, etc...
More thoughtful awesome films:
Powder
Phenomenon
City Of Angels