Karate Kid in a no brainer.
It epitomizes the 80's for me. The Reagan era optimism, the music, the fashion (check out Macchio wearing camo pants, with a TUCKED in flannel shirt with no belt) the dialogue (you couldn't just leave well enough alone, could ya, ya little twerp!). Everything screams 80's.
Beyond that, its a classic Campbell heroes journey and actually very accurate in its depiction of Martial Arts philosophy, if not techniques. Make no mistake, the fighting in Karate Kid is not good in a quantifiable sense. But as a story of a fatherloss son and a sonless father who find each other, and how the student can be just as important to the teacher, its unparalleled.
Runner up is Real Genius. I have so much affection for the tone of that movie. It wasn't nerds vs jocks, it was nerds vs other nerds, with national security and college as a backdrop.
Eddie Doty