Looks like I'll need to break them down in order to decide cause I just love all three picks too much...
Nightmare Before Christmas
I adore this film as a movie both for children and adults, that is a holiday classic while also being a bit dark with a fun story. Characters and music were so memorable that it's just as popular today as it was the day it came out, if not more so (I was a bit young to remember how hyped it was, but I know you can still get merch for it at any Hot Topic). But as a Christmas film, as much as I love it, I don't think I would give it my vote for best. Simply because, every year I don't know if I should even watch it for Halloween or Christmas. It focuses very much on both holidays, and on a story of its own where Jack is just trying to find himself. There wasn't really a Christmas-y message, and aside from the one song in Christmas town, I never really got a festive vibe from it. So... love the movie, but not my pick for best Christmas flick.
That was the easy one... Grinch vs Wonderful Life is a tough choice...
How the Grinch Stole Christmas!
I grew up with the Grinch. My parents grew up with the Grinch. I loved the animation as a child, and as an adult appreciate it for Boris's beautiful narration. The song is memorable and still played on the radio every year, and who doesn't know about the Grinch as a memorable character? The story a classic tale of turning a heart of stone to gold (or in this case, growing it), and a message rarely told, especially in today's world. Material possessions are just that, possessions. They're meaningless without the true meaning of Christmas behind them. A beautiful message that still warms my heart every time I watch it.
It's a Wonderful Life
I was in my teens when I had watched Wonderful Life for the first time. This movie really changed my life... Not only did it give me my first true appreciation for classic black and white film, but it had the best Godamn message I've ever seen. Similar to the butterfly effect, but in a more positive light. Everything you do, no matter how small, effects the people around you. No matter how small a good deed, it will be noticed and rewarded. Every life matters, especially your own, and is never wasted. I wasn't in a good place as a teen (most teens aren't), and I'm honestly still not (currently at an age where I'm still trying to find my place in this fucked up world), so whenever I watch this movie, it gives my perspective on my life a new view. Without feeling conceded I'm able to look back on the things I've done that might have benefited someone else, and I can feel a sense of pride. A relief that, hey maybe my life isn't total garbage after all.
If I were to answer the question for best Christmas film, I'd honestly have to say 'How the Grinch Stole Christmas'. It was the one movie of the three with the best Christmas message and theme. Wonderful Life may be a beautiful classic, but it didn't focus on Christmas a whole lot. It could have taken place any time of year and still have had the same impact really. I'll give it the point for best film of the three listed however because fuck, man... you just can't beat that message.
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Took me a while, but I think I came up with a satisfying enough answer. Sorry you have to dig for it though, Sculpt.