Although I'm not a sentimentalist, I do have a certain fondness for "It's a Wonderful Life".
I can't think of any movie that shows one man going through such a living hell - a two hour "long dark night of the soul" that's only relieved in the last five minutes of the last reel. This is one super-depressing movie.
This commentator at the NY Times has summed it up as well as anyone.
commentary at NY Times