My nomination for the Freethinker's best quote of all time:
"Ignorance is preferable to error, and he is less remote from the truth who believes nothing than he who believes what is wrong." --Thomas Jefferson: Notes on Virginia Q.VI, 1782.
Honorable mention goes to:
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain