The Keep on the Borderlands (or perhaps The Village of Hommlet) exemplifies low-level play, while the Giants-Drow modules exemplify high-level play. Likewise, Call of Cthulhu has Masks of Nyarlathotep and RuneQuest has Griffin Mountain.
While GDW's Traveller was blessed with many great adventures over the course of its existence, for me 1983's simply-titled The Traveller Adventure will always stand taller than any of the others. Written by Marc Miller, with assistance from many other GDW staffers and freelancers, The Traveller Adventure isn't a single adventure so much as the skeleton of an entire campaign, with lots of ligaments and musculature provided for the referee as he fleshes it out for use at his own table. Consequently, it's an open-ended, providing a sandbox-like environment in the form of the Aramis subsector of the Spinward Marches. Because of the nature of interstellar travel in Traveller, there are limits to where and how quickly the characters can travel and this provides Miller and his co-writers with the perfect means to drop rumors and hints and that might encourage the PCs to head in certain directions without resorting to more heavy-handed approaches. It's a good compromise between aimless wandering and railroading in my view and one I've often emulated in my own campaigns.
The Traveller Adventure does make a few assumptions at its start. In addition to its location within a backwater subsector, the adventure expects the PCs to be the crew of a subsidized merchant vessel assigned to a particular cluster of worlds all easily reachable by a jump-1 drive. For those unfamiliar with Traveller's conventions, jump-1 drives can travel a single parsec over a week-long journey through jumpspace. At the start of The Traveller Adventure, the PCs are "trapped" within a cluster of worlds all reachable by jump-1 but worlds located farther away (2 parsecs or more) require a ship with a better jump drive. Initially, there's no pressing need to acquire a better drive, but, as the adventure unfolds, that situation changes and the PCs must somehow acquire the funds needed to get a better drive, thereby providing them with a reason to take advantage of certain opportunities that come their way.
The central conflict of The Traveller Adventure concerns the efforts of a high-level megacorporate officer to get rich by illegal means. The PCs inadvertently become thorns in his side when they make the acquaintance of a Vargr (a wolf-like alien species) who himself has run afoul of agents of the same megacorporation. Should the characters help the Vargr -- if they don't, the campaign ends before it begins -- they'll slowly become enmeshed in a conspiracy that spans many worlds of the subsector and involves them in both megacorporate and political maneuverings they never suspected. Along the way, they'll visit many unique planets, interact with dozens of NPCs, and generally explore a small corner of the Third Imperium in great detail.
The Traveller Adventure is not flawless. Often the main thread of the adventure requires that the PCs go in a certain direction to continue and, while Miller and company are to be commended for not forcing particular actions through railroad-y situations, there is a danger that, in the hands of an inexperienced referee, the whole structure could collapse in on itself. On the other hand, The Traveller Adventure is a very fun adventure that rather nicely illustrates what Traveller is all about. The antagonist of the adventure is no black-hatted villain, but rather a venal executive chasing absurd profit and damn the consequences. Certainly innocents will die if his plan succeeds, but that's mere collateral damage rather than his actual goal. Likewise, failure by the PCs doesn't mean the end of the Imperium or anything so grandiose. In short, it's an adventure about people acting like, well, people, even if they live in the 57th century. I like that a lot, which is probably why The Traveller Adventure will always be the quintessential Traveller adventure for me.