The Digimon Frontier movie, The Revival of the Ancient Digimon.
Showcases in a pretty good manner the conflict between the Digimon and how an evil one profits from it by fooling both sides. Also, across the movie we can see the beautiful friendship between Bearmon and Kotemon so that's also something good from it.
Runaway Locomon and Children's War Game are probably tied as my top favorites.
I might be more biased towards Children's War Game because I saw Digimon: The Movie as a kid, so there's more of a nostalgia attachment with it, but in terms of art and animation it's very fluid and visually stunning.
Runaway Locomon feels like a really satisfying wrap-up for Tamers, and the tone is somewhat sentimental and bittersweet. It's also really well animated, and has some of my favorite character moments.
I'm not sure if anyone's mentioned it yet, but I did enjoy X Evolution. I like the fact it took a different route than usual, albeit a dark one. Though I think the visuals aged poorly, I did enjoy the story and characters.