For me, it’s got to be Back to the Future.
1. It’s got great script writing, obviously. But the fact that Part 2 arrived 4 years later, and picked up EXACTLY where 1 left off is just sweet as hell. You only get about 3 minutes of back story and that’s it. Same with #3. It’s impressively superb.
2. Even with a casting change from Claudia Wells to Elizabeth Shue for role of Jennifer Parker; it was almost seamless. I’m not saying that it’s not obvious, but Shue was able to deliver the lines and mimic mannerisms of the original that she was a great replacement.
3. It’s the ultimate science fiction, coming of age, underdog, comedy, serious movie I’ve ever seen.
4. The Timeline. Back to #1, the timeline stays consistent throughout the writing. Going from ’85, to ’55, to 2015, to Alternate ’85, back to ’55, then 1885 and finally ending in ’85. The writing stays consistent, and almost impossible to poke holes in. If you can find some, bring them up – let’s talk about them.
Check out this artwork for the movies, it’s pretty interesting the way the artist portrays the timeline: Here