My personal favorite from one of my favorite episodes.
"We think we've come so far. Torture of heretics, burning of witches, it's all ancient history. And then, before you can blink an eye, suddenly it threatens to start all over again."
I love this episode "The Drumhead". TNG: Jean Luc Picard
One of my favorite quotes is also from that episode. "With the first link, the chain is forged. The first speech censured, the first thought forbidden, the first freedom denied, chains us all irrevocably."