What type of query letter piques your interest?
One that's as well written as the book itself. Even better is one that's short, pithy, and demonstrates the author's understanding of, and aspirations for, how the book will be received as part of the literary and cultural conversation. I'm much more interested in knowing why an author wrote something and what kinds of books and authors inspired her, than I am in a lengthy synopsis. The latter should take no more than one paragraph.