Unlike many of those favourites lists or Top Ten-type questions that do the rounds at this time of year, I can give you my answer to this one straight away. It's The Bishop's Wife (1947).
I like to think of The Bishop's Wife as a minor seasonal miracle. It contains many traditional Christmassy things – Snow! Hats! Ice skating! Trusty servants! Mean old folk having seasonal changes of heart! Unresolved sexual tension! – yet somehow it manages to stay on the happy side of heartwarming without tipping over the safety barrier into a pit of schmaltziness. Most importantly, I first saw it when I was little, so a deep part of my brain associates it with cosy evenings on the sofa and warm bedtime drinks.
Basically, the plot is as follows. David Niven plays a bishop who wants to build a new cathedral. He prays for help, and an angel arrives in the form of Cary Grant. The angel knows, however, that the bishop's problems are nothing to do with cathedral-building and are all about learning to appreciate the good things in his life. He also turns his attentions to brightening up the life of the bishop's wife, played by Loretta Young. However, she's so sweet and lovely that the angel begins to fall for her too...
Sounds good? Well, here's a potted introduction to this merrily magnificent movie. (Be warned: this post does contain a few 65-year-old spoilers, but I don't think they'll seriously affect your enjoyment of the film if you want to get the DVD or hunt it down on Netflix. It's not exactly full of startling narrative twists.)
This is Cary Grant, who plays an angel called Dudley. He's suave, handsome and has a heaven-sent ability to charm the pants off absolutely anyone. (Not literally, I hasten to add. It IS 1947, after all.)
This is David Niven as the titular bishop, Henry Brougham. He prides himself on leading "a well-ordered life", but other people call him "confused, indecisive and totally ineffectual", and his idea of complimenting his wife is to tell her that she brushes her hair very capably. (Romance: you're doing it wrong.) When Dudley the angel turns up and says "Hello, I've come to help you sort your life out", his reaction is disgruntlement, accompanied by some fabulously flexible facial expressions. When the same angel proceeds to flirt with his wife, charm his housemaid and even hog the attentions of the family dog, he gets so peeved that I always fear his eyebrows may pop off.
This is Loretta Young as Julia, the titular bishop's wife. She is glowy yet sensible and glides around the place in a haze of soft-focus serenity (mind you, so would I if Cary Grant were constantly telling me how wonderful I am). She has no idea that Dudley is an angel, and thinks he's a new assistant who's been hired to help her husband with his ever-so-slightly Freudian cathedral-building obsession ("This cathedral must rise!").
Of course, Dudley hasn't come all the way down from heaven for admin purposes alone, but he's quite good around the office nevertheless. No, he's come to give the Bishop a more general sort of lifestyle guidance, and he elects to do this primarily by flirting massively with the bishop's wife (hence the title) and cosying up to all his friends in a more platonic way.
Being a girlie, I find the period version of sexual tension between Dudley and Julia very appealing. There are lot of longing glances from him and a lot of aren't-you-sweet semi-oblivious denial from her. At one point, they even hold hands in a public place, which causes a veritable uproar.
See? The middle classes are positively outraged. But of course, Dudley turns his twinkle on them and soon they are completely won over, as are all the other supporting characters who come into his orbit. One of my favourites is Elsa "Bride of Frankenstein" Lanchester as Matilda the bishop's housemaid, who goes into fangirl swoonage mode every time Cary Grant so much as glances at her. (Again, this is entirely understandable behaviour.)
My other favourite is Monty Woolley as Professor Wutheridge, a jolly atheist chappie who's suffering from twenty years of writer's block. Dudley turns the magic finger of inspiration on him AND gives him a magically refilling sherry bottle, which has got to be one of the best divine gifts EVER. I mean, if you knew God wanted you to have an endless supply of decent booze, you'd be much more likely to believe in him, wouldn't you?
If you're inclined to look for it, you can even find a tiny bit of Dudley/Bishop slashiness in there. After all, they're competing over the same woman, which is basically a fast track to homoeroticism whichever way you splice it, PLUS they do a lot of squishing through doorways at the same time. That said, this was an era where two characters can have a chat that runs: "What's that you're humming? It's rather gay." / "Well, I feel gay!" and nobody bats an eyelid.
(If you want a Cary Grant film with more slashiness, however, My Favorite Wife is a goodie. See here for more details.)
My favourite sequence in the movie, and the one I remember best from when I was little, is when Dudley the angel takes Julia the bishop's wife for a jolly skating trip, while a comedy taxi-driver called Sylvester tags along. Even given the massively unsubtle use of stunt doubles, it's somehow totally charming. And when Julia says "I feel as if I were doing something wicked", you may well wonder whether ice-skating is for angels what dancing is for Time Lords...
Speaking of Doctor Who, there's even a moment where Dudley laments in a rather Gallifreyan fashion: "I'm tired of being a wanderer. I'm tired of an existence where one is neither hot nor cold, hungry nor full." There's also a bittersweet ending: the bishop learns to appreciate what he's got, but Dudley must return to heaven and can never return, because his "superior officers" never send him to the same place twice: "We might form attachments." Awww.
The only real disappointment is that the film ends with a big old reset button: everyone forgets Dudley's visit and nothing remains but an angel dolly for the bishop's daughter and "the most inexplicable feeling of happiness". I've watched this film umpteen times and this bit disappoints me every time. Nonetheless, I suppose it's quite appropriate that there's no yelling or squealing or passionate declarations. Everyone's simply left with a vague sense that most things are okay. And at Christmas, that's probably about as good as it gets, wouldn't you say?
P.S. There's also a 1990s remake called The Preacher's Wife, starring Denzil Washington as Cary Grant and Whitney Houston as the titular wife, but I haven't seen it. Why mess with quasi-perfection?