The Nightmare Before Christmas (1993)
I find it interesting that both this and Elf share the same inspiration: both movies are set in worlds extrapolated from Rankin/Bass Christmas specials, and both take those settings surprisingly seriously. While Nightmare Before Christmas and Elf couldn't really be described as being in continuity with each other, either could easily be imagined in continuity with Rudoph.
At any rate, there's a long list of reasons for why Nightmare has become the classic it has. In addition to its ties to existing classics, it's brilliantly designed, beautifully animated, and the music is amazing. I'm always a little surprised by just how much of the movie is devoted to songs: there's actually very little dialogue or silence. To put it in context, I can't think of a single Disney musical with as high a percentage of song (unless, of course, you want to count Fantasia). Ultimately, Nightmare Before Christmas is closer to an opera.
I doubt there are many people out there who need to be convinced to see this. If you're trying to track down a copy, I suggest checking on your DVD shelf: it should be there. If you can't find it, you can order a replacement copy here.
* Past FIFTY years? Movies in COLOR? As of writing this, I haven't seen Miracle on 34th Street or It's a Wonderful Life in quite some time, and I have fond memories of both. Until I get around to those and watch my illusions come crashing down, I'm going to cover my bases.