This is a hard one for me to narrow down. Heck I have almost 200 movies on my iPod, ranging from Big Business
I tend to watch movies over and over again to go to sleep. I can't watch movies that I have never seen or haven't seen for a while because then I actually "watch" them.
My favorite movie varies from day to day, week to week, month to month. I suppose right now, anything to do with babies would be the theme right now. (No surprise there!) So I would have to say "Baby Mama" or "Look Who's Talking"