I don't know about the best, but the one's that stick in my head are:
"Si vis pacem, para bellum." (If you want peace, prepare for war.)
"A Roman does not wait for death. She seeks it out, and meets it on her own terms.”
Also, my mother once reminded me that though she is the goddess of war, she would never fight my battles for me or give me easy victories. Instead, she grants me opportunities to prove myself and gives me strong enemies and potential allies. Which has turned me into a better warrior and a better praetor.