If I may add my personal list of "classics" from 1st edition AD&D:
1. G1-3, D1-2, Q1: Giants, Drow, and Lolth herself! What can beat that? This is the epic adventure.
2. A1-4: The Slavers series was challenging and had players rely on their wits.
3. S2: Great cover art, cool magic items, and...