B2, Keep on the Borderlands.
If you don't like that, then T1-4 Temple of Elemental Evil or X1 Isle of Dread.
These have in common: 
(1) A starting town and base of operations
(2) Wilderness or overland areas to explore
(3) A dungeon that allows for multiple expeditions at increasing difficulty
(4) Some unique and memorable NPCs, encounters, and monsters
(5) A meta-plot, but one that is "soft" and allows the DM and players to develop their own motivations and story
(6) Non-linear structure, rendering player choices meaningful
The above list in my mind is what makes for a great introductory adventure module, and is a good model even for non-introductory adventures.  Later edition adventures with similar structure include The Lost City of Barakus.