I'm not sure I have a favorite movie, but if forced to answer the question, I usually refer to the Lord of the Rings project (which is technically more than one movie, but it's a more satisfying answer than nothing). It was an amazing effort, and the result was probably about as successful as anyone could have hoped for. But I'm really a fan of the books. I can enjoy the movies, and that says a lot, but my loyalty is to the original.
Can you blame me? I started reading LOTR when I was in fourth grade and averaged something like 1.5 reps per year until I finished high school (including the Hobbit and the Silmarillion). I traced maps, charted family trees, practiced runes, and imagined myself in the stories for countless hours while mowing the lawn. Those books were the gold standard not only for their own story but for just about everything else I ever read.
Fortunately, I was not quite such a fanatic by the time the movies came out. I was much less obsessed, and I think in general I had matured to the point that I realized a movie must always distort the book on which it is based. To a great extent, I could accept the changes made. They ran upwards of three hours each, with extended DVD versions even longer, but there was still no way they could include everything. And some interactions just don't work on film like they do on paper. I get that.
I probably hadn't watched the movies since 2004, when the last one came out on DVD. Sitting down to watch just one of them without the others seems silly, and carving out enough time to watch all three extended versions (alone, because the kids weren't old enough, and Julie wasn't crazy enough) is no small task. But for a while I'd been thinking about just starting to watch in bits and pieces and eventually getting through. I finally started the other day, and of course it didn't happen like that. I watched roughly one full disc at a sitting and finished in less than a week.
I was surprised to find that, not only could I stomach most of the changes, but I actually had that tingling in the back of my head when the elvish contingent showed up for the battle of Helm's Deep. It's spurious but moving nonetheless.
But the Scouring of the Shire. Jackson not only ignored it--he threw it hurtling from the top of Orthanc to a grisly death below. (Don't feel bad for Saruman--he may not make it to the end of the movie, but he gets an inflated role earlier to make up for it.) The discovery of Longbottom Leaf is retained (at least in the extended version), but is reduced to mere nostalgia instead of the ominous sign that it is in the book.
You could chalk it up to mere length, but as I recall they explicitly stated their reasons, and a significant motivation was that it just seemed to drag out the story anyway. The war is over, so wrap things up and be done with it. But this time watching, I had an epiphany. (I'm sure many have seen it already, but I'm slow with these things.) The Scouring of the Shire (and its absence) has a lot to do with the context in which the story is written.
If you don't know, the Scouring of the Shire is the penultimate act of the story. After the war ends, Aragorn is crowned and married, and everyone travels back home, there remains the task of mending the hobbits' beloved Shire. It comes out that Saruman was deeply involved with that part of Middle Earth, and his henchmen were running things at the expense of the hobbits. The movies get his obsession with machinery and progress; in the books this obsession wrecks the agrarian Shire even after the war seems over. The four hobbits return, Saruman meets his end, and the cleanup process begins.
Tolkien wrote LOTR in the context of WWII. As in most of Europe, the war did not end for England when Berlin fell. There was physical damage to repair, and there were lasting political changes as a result of the war effort (many of which, I would guess, Tolkien also thought needed repair). War had changed the home front, even if its worst effects were elsewhere.
But in 21st c. America (and although Jackson is from New Zealand, I still think it's an American movie), wars don't affect us here at home. Sure, you might get the occasional raid by a Black Rider or two, but the real combat is safely contained somewhere else. Our lifestyle doesn't have to change (if it did, the terrorists would win), our economy doesn't change (well . . . ). War is someone else's problem. The only negative effect is when soldiers return with their scars and have trouble assimilating back into "normal" life. And the life they return to is hopelessly normal. So, instead of the Scouring, we get a scene of the four heroes sitting in the Green Dragon, exchanging knowing looks.
Of course the Scouring seems to drag out the story needlessly when viewed from our perspective. Having a ravaged home after returning from war seems like a superfluous and cruel addition, not a predictable consequence. Tolkien couldn't write his story without it, because it was the stark reality of war. Jackson couldn't write his story with it, because if our wars change anyone's life, it's certainly not ours.