Wait, I'm supposed to choose one? Also, just how old-school? Listing a few out by the platform I originally played them on...
NES: Zelda 2 -- I still say it was better than the original. Rygar. The original Final Fantasy.
PC: Bard's Tale. Star Control -- though mostly because it spawned Star Control 2. Starflight -- though again, the sequel (Trade Routes of the Cloud Nebula) was significantly better.
Mac: The Journeyman Project (1-3). Myst (and Riven and Exile).
Apple IIe: Bug Attack. Frogger. Lode Runner. Some early iteration of Carmen Sandiego. Many Infocom games.
Arcade: Gauntlet, Magic Sword.
I could probably keep going.