For me, it was The Unforgettable Fire. Right from the first wave of ambient drums and blurred guitars to the dreamy synth and quiet (for Bono, at least) vocals that brought it to an end, I was completely stunned. I must've played it front-to-back at least 4 times the day it was released, and I still could barely believe that, "Pride" aside, it was even the same band.
your turn.