I'll start - S02E09 Measure of a Man

This is my favorite TNG episode for a couple of reasons. First, it shows that they didn't need amazing effects and set pieces to drive the story - most of this ep consists of the actors sitting in a single set and talking, yet the story is still fantastic and the episode is riveting.

Next, this is my favorite because Data is my favorite Star Trek character overall, and I feel this episode is a key piece of his character's development.