For me it would have to be Dominique Dunne, back when I was about 18 years old. I had been following her career (albeit short) for a while, and had just seen her in Poltergeist, when I heard that she was on life support. When I asked my sister, who was studying to be a nurse, what Dominique's chances were of recovery, she told me she was pretty much dead, there was no hope. I was completely crushed.
The last time I thought to myself: "I can't believe they're dead" was in regards to John Ritter. Actually, about every six months or so I think about him being gone and it just seems surreal. I know that probably sounds sappy, it's just that he seemed like such a good guy.
Here is a great tribute to him written by Larry Miller. About midway through, Larry talks about the day before John died. Very touching.
http://www.weeklystandard.com/Utilit...3133&R=EEB0A8E
Now I'm all verklempt...talk amongst yourselves.