Probably Harry Potter and the Sorcerer's Stone. Between Harry Potter Weekends and the fact that it lives in my DVR as my "I'm upset or otherwise can't fall asleep movie," I've watched it a lot!
Might be The Sound of Music, though. I didn't have cable in college, but I had a VCR (and a couple of movies nobody at home minded me taking) and that was my go-to background movie.
Oh, and Winnie the Pooh when DS was little! (I got so tired of that one, I went into the DVD menu and started playing it in French.)
I think mine is "The Empire Strikes Back" but it's not my favorite movie. In fact, I'm pretty sick of it.
It's my husband's favorite movie and he watches it a LOT! If it's on TV he watches. It's his go-to entertainment if he's feeling sick. Just about anything is an occasion to watch it. I will say "Didn't we *just* watch this?" And he will reply "no... that was the such-and-such cut. This is a different cut. He changed X, Y, and Z in this one. [insert rant about not being able to get the original theatrical release on Blu-Ray.]"
The Way, Way Back. Absolutely love it. As does my entire family. We watch it whenever we need a break. Sometimes that is once a month, other times, once a week..
A Christmas Story and Christmas Vacation are probably the main contenders.
Also, National Treasure and the Jackie Chan remake of Around the World in 80 days are family favorites. I also watch Under the Tuscan Sun and The Thomas Crown Affair a couple of times a year (these are all on my iTunes).
When the kids were little, Barney's Imagination Island and Aristocats got worn out in the VCR. High School Musical was DD's favorite as well.
My husband doesn't understand it but I enjoy watching tv shows for storm chasing--now the devestation is terrible however so I don't enjoy those parts of the shows.
Footloose, Dirty Dancing, Twister, Harry Potters, Contact, The Muppet Christmas Carol (don't hate but it's the version I grew up with and is my fav version), The American President, The Santa Clause, Center Stage, and I'm sure more lol.