The original Star Wars trilogy
Miracle on 34th Street
Rocky (though I did recently see Rocky Balboa)
The Dark Knight (I didn't think much of Batman Begins)
Raiders of the Lost Ark
There are probably more. I don't really make an effort to see these things. But sometimes people take it into their own hands....
Movies people have made me watch because they were shocked when they learned I hadn't seen it:
It's a Wonderful Life
That's all I can think of off the top of my head. Anyway, I figure if people are truly scandalized that I haven't seen something, they'll make it happen.