Best 'best' is hard to me to say; a lot of people don't like, but i say "inner light". As Picard receive the flute back and start playing, the emotional weight of the scene got me good.
That episode hits so hard if you think of the idea that Picard sacrificed basically everything for his career. He never married, never had a family or settled down on some backwater planet.
And then for a lifetime - in 27 minutes - he did. He got to have the life he never got to have.
It just. My soul hurts for him. I still don't know if it's sad or beautiful or both but that episode tears my heart out every time.
The Inner Light was what finally got me into Star Trek.
It's not like I had never seen an episode, in fact I'd seen lots, but it was comfortable background noise I'd change the channel too as yet another repeat I had no context for aired. Always somehow a filler episode or the second part of a multi-episode arc. I'd written off Trek as pleasantly mediocre and wholly dependent on technobabble despite otherwise being an extremely keen sci-fi fan.
Luckily, on some long forgotten forum, someone described an episode which sounded nothing like the Trek I had seen. As you said, the emotional weight got me good.