I guess "Jump the Shark" choice depends on your definition. The show's best moment to me was the end of "Reunion," so everything was technically downhill from that point (though still very very good). There was a marked drop in quality with the Pylea arc, which ended the series' golden age. But I agree with Hec that the Crapture in "Tomorrow" was the absolute lowest point.
Given that I don't think anyone considers the shark jump episode of Happy Days to be its zeneith, I've always used the term for the "abandon all hope" moment for a show - the point where it's never going to be any good ever again. The crapture came close, but there were enough fun season 5 episodes to have me think that it ever really reached the point of no return.
For a good example, I've always liked the X-Files as a show that once it reached a certain point, there was no reason to keep watching. I'm tempted to say that point was the movie, and how the season after seemed to be trying to contradict/take back/retcon everything the movie seemed to tell us.