I am ashamed that I did not know Kubrick did Spartacus. Tremendous movie.
However:
1. 2001 -- perhaps the most thought-provoking movie of all time, even if you're just trying to figure out what the hell it was about.
2. Dr. Strangelove -- coming on the heels of Fail Safe, it make everyone find something funny about the Cold War and Nuclear Armageddon.
3. Full Metal Jacket -- "Whassa matter, Pyle, didn't mommy and daddy pay you enough attention?!?!" (Gunshot)
4. Clockwork Orange -- after 2001, perhaps the most brilliant movie of its day.
5. (Tie) Paths of Glory/Lolita -- made us examine what we had assumed was always true and right.
Honorable mention: Eyes Wide Shut -- terrible movie, but Nic Kidman sans panties. Oh my!