If anyone of you hasn't heard of this before...
http://en.wikipedia.org/wiki/Reply_of_the_Zaporozhian_Cossacks (http://en.wikipedia.org/wiki/Reply_of_the_Zaporozhian_Cossacks)
I think that is the most badass letter written in human history.
I prefer (mildly) the Spartan reply to Philip II of Macedon.
Philip: "You are advised to submit without further delay, for if I bring my army into your land, I will destroy your farms, slay your people, and raze your city."
Spartan response: "If."