Citizen Kane. Good Lord, why is that such a 'good movie'? It is one of a very few movies I have ever walked out on. I was told one reason I hated it is that I knew what Rosedbud was way before I saw it.
Vertigo is awful as well. Blah. How booring. Another one I walked out on.
I picked these two because they are supposedly classics. They suck.
If you want classic, see something worthwhile, like Cassablanca, Plan 9 From Outer Space , or Harold and Maude. Those are classic movies, all for VERY different reasons.