The Deer Hunter.
First time I watched it I was spellbound, totally mesmerised. Cried myself to sleep. Stayed in my mind for years - even now when I see Christopher Walken I get flash backs.
Second time it was like post traumatic stress disorder. Watched the first act, then as they landed in Vietnam, I got so upset and panicky I had to turn it off. Just couldn't watch it again. That was about 10 years ago, so I'll have to give it another try. I hate to think I'd have to spend the rest of my life never able to watch what I would argue is one of the greatest film of all time.
Oh, and 'Welcome to the Jungle'. Because it was a big pile of poo. And boring.