A great way to approach this letter is to write it from the perspective of having finished all of your interviews. You're looking back, thinking about all of the programs you visited and people you met. Why does this particular program stand out for you? Why exactly do you want to train there? What should the residency director realize that you have to offer the team at that program?
Do *not* tell multiple programs that they are your number one choice. Although NRMP rank lists are never made public and it may seem safe, lying to a residency program could be discovered quite easily if a program has ranked you high enough that you would have automatically matched there had your statement been true. Residency committee members can have long memories, and when you are applying to fellowship or attending positions, submitting articles to journals where they serve as reviewers, attending conferences, or interacting with them in other capacities, I would hate for them to remember you in a negative light. It's even possible for the program where you *did* match to discover that you had lied to other prospective program directors, leading to an uncomfortable discussion about ethics.
If you send this letter via email, and you're wondering what to use as the subject line, many of my clients use one of these:
- letter of intent
- letter of interest
- ranking plans
(There's no official rule about what to title the email, so if you want to use something different, that's fine. Many people don't pay close attention to subject lines anyway.)