I can't tell you how many times I've seen Jaws -- just not from the beginning. If I discover it's on, it's like a magnet, no matter how far into the movie it is. And I own the DVD, it's not like I can't watch the whole thing any time I want.
Same way with Psycho. And most of Hitchcock's movies.
The Third Man is another one, as is Touch of Evil. And in a completely different direction, Guess Who's Coming to Dinner.
Christmas movies: It's a Wonderful Life, Christmas Carol, A Christmas Story, Miracle on 34th Street (original), Polar Express.
More contemporary movies would be The Green Mile, Twister, any of the Bourne movies (although I always have to figure out which one I'm watching), the last Godzilla remake.
Or, just about any 50's sci fi movie, both the good ones, and the bad ones.