Imagine you're back in high school, planning to ask out a fellow classmate to the prom. You approach your potential date, "Would you be interested in going with me to the dance? You're my second choice."
Not very compelling...The same is true for letters of interest or, really, any outreach to medical schools or residency programs: Over the years, I've edited many letters in which applicants mention that an institution is "one of my top" picks.Avoid that type of language. Instead you can say something like "I would be thrilled to be at your institution," or "I am confident I can make a positive contribution." In other words, you can stay honest while not shooting yourself in the foot.
Let me know if you need assistance with a letter of interest.