A Man, A Plan, A Canal: My New Movie

Q. What should be my subject line when I am sending query emails?

You want the victim recipient to open the email, right? Put the hook in the subject line. At least, as much of it as you need in order to get them to open it.

SUBJECT: A man discovers he has been replaced by his clone...

Or, if you have a great title, put that:

SUBJECT: Nick and Norah's Infinite Playlist...

Or, the theme:

SUBJECT: Can men and women ever really be best friends?

Actually, that last one reads like spam. And so would this:

SUBJECT: I know what you did last summer...

So you need to clarify that:

SUBJECT: Can men and women ever really be best friends? WHEN HARRY MET SALLY... a comedy script.
SUBJECT: I Know What You Did Last Summer... a horror script.

I guess the real answer to the question "what should my subject line be?" is whatever gets someone to read your email!