The end. The pay-off. That’s what makes a story worth it.
If that most important piece is not handled properly, any investment by the audience is lost. They could have spent real time becoming invested in the characters, plot, and themes only to be let down when the weak link becomes revealed.
A good story is a puzzle, and it is the writer’s job to make sure that every piece fits together well.
Start with the end. It’s where the audience ends up. Make sure they are satisfied.