Mine is probably Gone With the Wind. But then I wonder, despite it being a classic and one of the most (albeit controversially, now) highly-regarded films of all time, has "everyone" actually seen this?
Pretty sure it was on TV and maybe my parents watched it, but I'm quite sure I never sat through it all.
What I am completely at loss about it's E.T. - it was so long ago and scenes from that movie are part of pop culture that I truly and seriously don't remember if I watched it when I was a kid, or I just know the pivotal scenes "everybody" knows.
Actually in later years I made a point to try and catch up with famous old movies - saw the Godfather trilogy, Psycho, 2001 A Space Odyssey, Casablanca, The Last Emperor, Gladiator, Once Upon a Time in America just to name a few.
Browsing through the list of Best Movie winners at the Oscars, other very famous titles I didn't see are:
Shakespeare in Love
The English Patient
The Deer Hunter
One Flew Over the Cuckoo's Nest