All of the listed text features can help readers identify the main ideas in an informational text, but if you have to choose one that is particularly effective, it would be headings and subheadings. They structure the content and give clear indications of the main topics being discussed.
That being said, bolded or italicized vocabulary words, introductory and concluding paragraphs, and captions under images also play important roles in highlighting key concepts and providing context, so they should not be overlooked.