Late to the party, but I nominate Season 4's Half a Life.
While traveling on board the Enterprise, Lwaxana Troi encounters a soft-spoken alien scientist named Timicin (the always excellent David Ogden Stiers), who is working with the crew to save his planet's sun. Despite their differences, love blossoms between them. All is imperiled, however, when Troi learns that Timicin has reached the age of "The Resolution" among his people, and is expected to commit suicide in order to not burden society in his old age.
Why I Nominated This Episode
Half... is, IMHO, one of the most thought provoking and thoughtfully done episodes of the series. It addresses a topic of a serious moral and ethical nature and does so with aplomb. That alone sets it apart from most of TNG's "message" episodes.
As much as I loved TNG, it often handled such matters awkwardly. The results ranged from merely hamfisted, to insufferably annoying in its smugness. (Riker smugly virtue signalling to the Antican ambassador about replicated meat, Counselor Troi smugly poo-poohing "outdated morals" in the "Outrageous Okona" episode, and so on. And then there's Tasha Yar's fetal-position inducing Level 5 Smug Alert anti-drug speech to Wesley Crusher...)
Half...commits none of these sins. It takes the subjects of aging and the value we place on our elders, and embraces them without resorting to cliche or partisanship. The resulting storyline is nuanced, with both societies' views well-represented without the show taking sides. It makes for first-rate, and rather profound story-telling.
Equally worth mention is the superb acting in the episode. In particular, Majel Roddenberry's standout performance.
I'd never liked the character of Lwaxana Troi in any of her previous outings. In fact, I loathed her appearances. I never found her amusing or even so-bad-its-good amusing. Just incredibly irritating and a waste of screen time. Half a Life completely changed my mind about the character, and Roddenberry's own acting abilities. Majel is sublime in the episode. She's passionate, fierce, and absolutely believable in her convictions against Timicin's planned suicide. It's great stuff, and she proves to be every bit as compelling on screen as Stier's Timicin.
The only other thing I'd add is, this episode marks the very first appearance of Michelle Forbes on TNG. She plays Timicin's daughter, Dara.