"Yesterday's Enterprise" is the first episode from Next Gen that I remember... Next Gen had been on "in the background" during my college years and I didn't pay much attention (I wasn't an original series fan), but that episode made an impact, and the next year when a friend talked me into watching all the season 1-3 eps he had taped, I was willing to because of that episode.

One of my stories that didn't make it into the Strange New Worlds anthologies was related to "Yesterday's Enterprise." For me, the other "big impact" episode was "Lower Decks," although ultimately DS9 edged out Next Gen for me.