If you read nothing else: the headline is not one part of the landing page. For most visitors, it is the entire page. They read it, they make a judgment, and they either scroll or leave. Everything below the fold — the testimonials, the feature list, the trust badges — exists to support a visitor who already decided to keep reading because the headline earned their attention.
A great headline does not need a great page to convert. A great page with a weak headline will barely be seen. Write the headline first, test it against the four-second rule, and only then build the rest of the page around it.
For a library of formulas you can use right now, see hero headline formulas — and to find out whether your current headline is the leak, run a free landing page audit.