The denotation of a "most important passage" in any text can only be considered objective. When reading, people come to texts with different backgrounds, morals, upbringings, and ideologies. Therefore, each reads the story in a different way. To denote a singular passage as the most important in the text is impossible.
Therefore, an important passage in the story "The Cold Equations" is:
He leaned back in the pilot's chair and drew a deep, slow breath, considering what he would have to do.
This passage from the text shows the fact that Barton knows that something is going to happen. He recognizes the fact that whatever does happen, it will not be something that he can take lightly. He will be challenged; he will be pushed; he will need to contemplate many different things before making a decision.
This passage allows readers to define one aspect of Barton's character--the author has given readers an indirect clue as to the type of person Barton is. Barton is not one to make snap decisions. He is not one to take anything lightly. He is a "considerer", a "contemplater."