What is your Favorite National Monument?

I wouldn't call it a "favorite" as that would seem a little off color, but I found the Vietnam Memorial (I have not visited any of the other military memorials in DC) to be one of the most tranquil places on earth. The peace and quiet that mixes with the overwhelming sorrow and respect is just amazing. It's a time for reflection and thanks to those who give their lives every day for our freedoms.