The perfect pitch is just as important as the perfect first sentence, paragraph, or chapter. Without the right pitch, your manuscript could sail off into the outfield despite how brilliant it might be. Effectively communicating what your book is about, and why it's brilliant, is the key to getting an agent or editor interested enough to read beyond the query, or ask for pages after the pitch session.
In a query letter (to me in particular), you should start with a sentence that says the genre, word length, and why the manuscript is right for that agent/editor (this second part shows you did your research on them and helps personalize it). Some like to put this at the end but I like to see it right off the bat. The next part is the one-sentence hook. This is basically a summary of your book in one sentence. This does not mean you need to cram it all into a long run on sentence, the opposite in fact. It needs to be under 50 words or so (Twitter length is a good rule of thumb), highlight what is most interesting about your novel, and make them want more.
Following that are two to three short paragraphs highlighting the remaining elements in your novel that are interesting/important. Please remember, this isn't a grocery list, make it flow and read interesting. Think of the voice of your book and try to capture that a bit when you write this part. The elements that must be present are the main conflict, stakes, and resolution (what they must do) of the novel.
Sum it up with a short paragraph of relative information about yourself including social media links, a bit about how you've prepared your platform (social media following, newsletter subscribers, street team, etc.) and give multiple ways to contact you. Now comes the study in patience because responses can take a while.