The old Saltmarsh series — AD&D adventure modules U1 to U3, by Dave J. Browne and Don Turnbull of TSR UK — are far and away the best set of adventures I’ve ever DMed.
“Sinister Secret of Saltmarsh” is a perfect example of how to create a multilevel mystery narrative and marry a site-based adventure to a story without railroading the characters. “Danger at Dunwater” (besides having the coolest AD&D cover art ever!!!) is a great lesson to PCs and players alike about why you should never take a straight-up hack-and-slash dungeon crawl at face value. “The Final Enemy” marries a stealth infiltration mission to monstrous political intrigue and moral dilemma. Oh, and your characters are underwater the whole time.
What are you going to do?