If we're going really old school, then Space Invaders. Its way of leveraging the hardware at the time to make the enemies and music speed up after you defeat more of them is elegant. Back then, the more things a game had on screen, the slower it ran. So, destroying more enemies removes more things from the screen, causing both enemies and music to speed up.

This is something that's taken for granted today, but I think at the time, it was genius.