"Tomorrow Never Knows" by The Beatles in Mad Men (S5 E8)

Worth every penny of the quarter million dollars they paid to use it. Don tries to shut off the cultural revolution unfolding before his eyes but of course, it's all done in vain. Reinforced by the track picking right back up after the cut to credits, what has began will only continue and soon he'll find ways to adjust.

Runner up pick for me was another astounding song "I Just Wasn't Made For These Times" used to soundtrack another cultural revolution of sorts — Roger taking acid for the first time.