At work a couple weeks ago, I started whistling the music from WEAPON's attack on Junon. My coworker turns to me after a few bars, and says "oh my gosh, I just got a total flashback to that scene. I know exactly the moment, I remember how it played out..."
With just a few whistled bars, I brought back vivid memories of an entire chapter of a game.
That is what this thread is for. Not for the best, or favorite songs, but for music so inextricably linked to a moment in gaming in your mind that hearing it immediately draws you back, even if you haven't thought about or played the game in years.