"Donna Martin graduates!", while not a JTS moment for me, is interesting in that, watching it as a kid (I would have been a freshmanhigh school), I was totally on the side of the kids, but looking at now as a guy in his mid-30s...Donna should have gotten her barely-covered-in-short-shorts ass suspended. And Brandon too.
For me, there was no actual JTS moment. My favorite era of the show probably remains the summer after season 2 up until the end of season 5. I still watched the last 2 years of college, and then somewhat lost interest during season 8. I'd guess I bailed on the show sometime during that season. I did tune in for the final episode, and didn't watch the rest of the series until years later. The only thing I can say I really missed during that time was Steve and Janet. Much of the rest, like Noah, Gina, and Matt I didn't feel about not seeing during the shows first run.