The key pages are those that are high-value and as a result are most likely to be used. This is a subjective question, but there are some pages which will always be good candidates. These include the homepage, about page, contact page, and product pages.
There is no one answer to this question. What is high-value content for one person, might not be valuable for another. The key to determining what pages are the most important to your site is to think about what pages bring you the most traffic and conversions.