In theory, the subject line should be the simplest part of the query process.
And it is- as long as you don't overcomplicate it.
Say my book is titled "The Adventures of John Smith."
Typical wisdom would have us include a email subject line that says
Query: The Adventures of John Smith
Easy-peasy. Of course, it's always good to read the directions of the agent your querying. Perhaps, they prefer
Query: The Adventures of John Smith Attn: Agent of my dreams
The Adventures of John Smith, YA Adventure, 98K
What we shouldn't be seeing is:
The Adventures of John Smith- the best book you've ever read
Want to make $$$$$: Rep The Adventures of John Smith
For a limited time only: For your 👀's ONLY- The Adventures of John Smith
While a large part of the query is selling your book, you want to be carefully about the impression you leave on your prospective agent. Above all, you want to come off as professional, confident, and as someone who can follow simple instructions.