For an adjective that points to the main or most important thing, your best choice is principal. Is your principal goal for the summer to have fun or to earn some extra money?
The word, principal, comes from the Latin principalis "first in importance" and is related to the title of prince. You can see the relation when you think about its meanings — the head of a school, the starring actor in a film, and the client of an agent. Don't confuse principal with its homophone principle, a moral rule or law of nature. If you talk about a country's principal exports, you mean the main ones.