My initial gut reaction is to say one of the 1E modules; there are so many good ones to choose from. However, I've never run any of them. Back in the day, I wrote up all my own adventures. It wasn't until much later that I started running pre-made adventures. However, my overall favorite, that I have actually run, has to be the Vault of Larin Karr. This was a 3E adventure/setting from Necromancer Games.
For those who do not know, this book included a broad mix of setting and adventures. The setting featured a valley complete with villages, towns and other locations players could make into their bases of operations - and also locations that needed protection. Spread around this valley were various dungeons and other adventuring locations waiting to be explored, including a complete system of underground passages that ran the length of the valley. Along with standard dungeon fare there were personalities to interact with and puzzles to challenge the players.
Vault of Larin Karr was a large sandbox ripe for a long campaign. While there was an over-arching threat there was no singular way to go about keeping the valley safe from the machinations of evil. It was an excellent setting waiting to be explored.
I really enjoyed running the setting/adventures. It allowed me to entertain my players with a wide range of different styles of play (rping, puzzles, dungeon crawls) without me having to do all the preliminary work (other than reading the material). I would certainly recommend it for any D&D system.
Side Note: My second favorite is The Lost City of Gaxmoor, a 3E Troll Lord Games sandbox adventure. It was also a lot of wild fun and definitely evoked my old-school sensibilities.