But everyone else has seen it!

[Big D]

Mary Poppins
Top Gun
Breakfast at Tiffany's
The Breakfast Club
Godfather series
Rocky series
Rambo series
Mad Max series
Pretty Woman
Lawrence of Arabia
Ben Hur
Full Metal Jacket
Scarface
Children of the Corn series
Friday the 13th series
Aliens series
Predator series
Dirty Harry series
The Good, the Bad and the Ugly
A Fistful of Dollars
...any other Clint Eastwood western

Edit: Other movies people have mentioned that I've never seen either:
Evil Dead series
The Sound of Music

Basically, if you can name an "important movie that everyone has seen", then chances are I haven't seen it. Except for those few that I have seen. It's fun for scaring/annoying friends.