If you want to go to a place, telling them that you are ranking them number one is just fine. It does not break any rules to give them an unsolicited statement. Just remeber that it is a small community and if you tell a bunch of people in an unsolicited statment that they are number one and don't mean it, you may have some issues.
However, if any program asks you what you are going to rank them, you should have one answer..."number one". This was the advice that our old chairman used to give students. Reasoning behind this is that if they are not going to play by the rules, neither should you. No program should ask you for a commitment on your rank list (this is not legal in the match system).
It is really early to even think about sending these letters out. You should have almost all of your interviews done before even thinking about sending the letter. In my opinion, a "you're #1" letter now just makes you look desperate.