It's easily TPM in a walk. When the teaser came out, most people were still on dial-up, and internet video was a novelty, but people were crashing websites left and right just trying to see it. Star Wars fans were buying tickets to Meet Joe Black just to watch the trailer, and theaters reported seeing as much as half the audience leave after the trailer aired. When the movie came out, people were playing hooky from school/work ("Jedi flu"), people were cosplaying at showings (this was back when cosplay was still in its infancy and WELL outside the mainstream), and Fox sold out almost all the midnight shows back when almost no other movie even had midnight premieres. TPM was a HUGE cultural event.
Of all timePhantom Menace.
I went to a Thursday midnight showing and the air in the theater was practically electric when the lights went down. A roar of cheers went up when the "A long time ago" bit hit the screen. When people were leaving at the end, it was dead quiet. People were in shock. Some were crying. It was surreal.