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.
Measure of a Man is hands down the best episode of TNG.
LOL I started writing this having only seen your title. I happened to glance up and you beat me to it. You're right that Data is the best vessel that TNG has to explore humanity. Data shows us things about ourselves that we tend to take for granted. This exploration of what makes sentience and consciousness really shaped how I think about AI and all the current debate we're currently having. It makes me feel better that we don't have it all figured out in 2023 since they were still debating it in 2365.
The whole concept of Riker being forced to argue against Data is excellent. It shows his character for so many different reasons. The conflict he feels as a Starfleet officer and also as Data's friend, as well as the inner contradiction of needing to do his best to argue against his friend in order to save him, is some of the best Star Trek, IMO.
I loved the scene where he pulls up data's schematics, studies them, smiles in satisfaction and then visibly sobers with the realisation that his discovery might just cost his friend their status as a person