From the ones I have seen, the most tragic case was Two and a Half Men after Charlie Sheen stopped working for the show. With Ashton Kutcher, the show was completely different and nowhere near as funny.
Of the shows you have seen in recent years, which one had a good beginning and a crappy ending?
I think the anime Baki. Had high hopes when I read the anime description. Turned out a huge waste of time filled with unfulfilled expectations; thought there'd be DBZ level of action. Instead, we get what looks like a monologue by every damn character written by a 9-year-old that occupies 75% of an episode.