I'm not saying they're better movies, but I would say Back to the Future II and III are the gold standard in terms of planned two partners that still tell a complete and satisfying story in the individual films. Yes II has a cliffhanger but it's a tag on the end, the main story is completely resolved. I personally would not say the same of the LOTR films, certainly not the first, although that is by design, because the books aren't that way either (I believe they were intended to be published as a single book)