Help with resignation letter

Im resigning my current position with my agency and debating on who to address my resignation letter to...branch administrator or nursing supervisor...both of these wonderful nurses have been awesome and supportive during my time with the agency and i dont want anyone left out, but what is protocol on this process?
I believe that would be the nurse manager or the nursing supervisor. Whichever is your immediate supervisor that does your normal scheduling.
Your manager. Dear Manager, I resign my postion effective xx/xx/xxxx. Sincerely ... No need to point out their shortcomings, your lifestyle issues or the state of the nursing profession. If they ask you why, and you feel like sharing, go ahead. But if not, just say you don't feel it is working out. This letter is important for the future if they mistakenly state you were fired or any such other thing.