Originally Posted by
Stan
If not the first, the first famous sandbox module was the Isle of Dread, which was played by most players in the 80s because it came with the expert set. It has a nice mix of mini-dungeons or adventures along with sandbox wandering into trouble. It is a wilderness adventure, though, and not a dungeon crawl though it might be hard to apply to your own adventures.
The Keep on the Borderlands is a classic module, also boosted in popularity but being in a box set. It's nice is that it has a home base, stuff in nearby areas, and a series of small dungeons that are each good for a session of gaming. There are things you can use from the wilderness and keep for more complex plots if you want or you can just use it as a place to rest and buy stuff. There's only modest interaction between the caves but it's still an excellent module. The late 2e remake Return to the Keep on the Borderlands redid a few things and depopulated the keep somewhat and is also good.
I like In Search of the Unknown because it has some weird things that aren't just fighting stuff. I like mysteries that make players think but some people dislike effects without an explanation of how they could be created with the existing rules.
The Temple of Elemental Evil is potentially interesting with conflicting factions but I found it a long slog through a big dungeon that got tedious.
I've read only a few 3e and 5e modules but some were good - I'm blanking on names at the moment.