From our perspective, there are two good options for your subject line. Often, the hiring manager will indicate the exact verbiage for the subject line in a posting as a test to see which candidates can and cannot follow directions -- an easy way to weed out unqualified applicants. When there's a specific directive, follow it exactly -- if they used all caps or a colon, you should do that too. In this case, if you choose to go with your own subject line, don't be surprised when you don't get a phone call for an interview.
When there are no specific subject line instructions in the posting, we like to use “[Position title] Candidate: [First & Last Name]” (for example, “Agency Assistant Candidate: John Smith”). Using this format allows you to clearly indicate what position you are applying for, and it also helps you stand out in the crowd. Many email programs tend to group emails that have the same subject line together, and using your name will ensure that you will get your own thread. Plus, it makes your email easily searchable later on.
Realistically, your email subject line won’t make or break your application (unless you have a spelling error or fail to follow instructions). There are plenty of options that are perfectly acceptable, so pick one and then focus on creating a stellar cover email!