What's your favorite "preachy" Star Trek?
Trek has always had a penchant for didactism throughout the franchise, some cloaked in heavy allegory, some more direct, some just enough with the details smudged, whether it's Let That Be Your Last Battlefield or The Outcast or Voyage Home. Some consider it preachy, others consider it the heart of the franchise.
My favorite remains Undiscovered Country, this wonderful story that challenges us as a Cold War audience to try to do the same as the crew in overcoming old biases, the collapsing Soviet Union for us, the collapsing Klingon Empire for them.