For me, the hardest part of a query letter is the hook. Okay, that's a lie. The hardest part was summarizing the entire plot in a few tightly-articulated sentences that had to showcase enough personality to set my summary apart from the hundreds that agents and publishers receive every day. And honestly I wasn't very good at it at first. But the hook was something I thought about constantly, revised it one hundred times every time I took a shower, tweaked it while I brushed my teeth. I submitted over 40 queries in batches of 5-8, changing the query letter (and definitely the hook) with each batch.
The disclaimer here is that they did not all necessarily evolve in keeping with Darwin. I kept trying to make my hook about my protagonist - that she had a gift with horses (snooze), that she had a secret (who doesn't), that there was a boy who'd give up everything for her, even love, to keep her alive (well there better be!) And I wasn't getting any bites. I read several great blogposts about query letters and hooks (see below for a list), and realized that I wasn't using my hook to point to what makes my story unique. Truth be told, my query letter that minimally featured my protagonist is the one that got the most bites. The writing sample will likely showcase your character, but your query is a great place to showcase the conflict. As literary agents Mary Kole and Kristin Nelson have both said during different webinars: "Make me care!" What gives the story a sense of urgency? Why should you care if the worst case scenario goes down? For "Be Seeing You," if Tanzy and company collide with worst-case-scenario, this world of ours is going to be sucked through a tiny trap door and spin all of us into elemental mush. And so came my final hook: "Have you ever seen what's left in the bottom of a test tube after it's been spun through a centrifuge?" Now that's different, and it got a bite with almost every query that I used it in.
I feel compelled to say that a hook is as individual as we are - some people like to go the question route, others like to drop a bottle of wine on a tile floor (a BANG start, if you will) but for me, I like a start that creates an invitation. Step inside. Take a look around. I also feel compelled to say: read what the agents want in your query. Sometimes they're very specific - like one who asked for 12 point Georgia font double spaced. Another one said: don't sweat it, just tell me about yourself and what you wrote about. So make sure you learn as much as you can about who you're querying and what they're looking for.
Other blog posts about queries/hooks: