"The Drumhead" (4.21). My pick for the single best TNG episode. It's not simply the thesis ("witch hunts
are bad") with which we all agree already.
It's the depiction of how witch hunts grow, step by step, and how good
people are gradually corrupted. The
episode is gripping in itself, but it also forces us viewers to confront our
own demons… our potential to become self-righteous, to grow suspicious, to
judge others only at their worst.