I doubt I can pick just one. Here are some of my favorites:

"I may not agree with a word you say, but I will defend to my death your right to say it." - Voltaire

"Doubt is not a pleasant condition, but certainty is absurd." - Voltaire

"History is a pack of lies we play on the dead." - Voltaire

"A witty saying proves nothing." - Voltaire

"Those who can make you believe absurdities can make you commit atrocities." - Voltaire

"Think for yourselves and let others enjoy the privilege to do so too." - Voltaire

"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones." - Albert Einstein

"It is a miracle that curiosity survives formal education." - Albert Einstein

"Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." - Albert Einstein

"To punish me for my contempt for authority, fate made me an authority myself." - Albert Einstein

"Common sense is the collection of prejudices acquired by age eighteen." - Albert Einstein

"Insanity: doing the same thing over and over again and expecting different results." - Albert Einstein (attributed)

"The significant problems we have cannot be solved at the same level of thinking with which we created them." - Albert Einstein

"Great spirits have always encountered opposition from mediocre minds. The mediocre mind is incapable of understanding the man who refuses to bow blindly to conventional prejudices and chooses instead to express his opinions courageously and honestly." - Albert Einstein

"The unleashed power of the atom has changed everything save our modes of thinking and we thus drift toward unparalleled catastrophe." - Albert Einstein