The greatest D&D adventure path, was the first one.

 

Dungeons and Dragons modules have come a long way for good or for ill, preferences may vary, from the days of B1 and B2. Side note, B2: Keep on the Borderlands is a near perfect module and any perspective DM should cut their teeth on that one. Back where we were: today's adventures look much different than those of the early days of D&D. Gone for the most part are the sandboxes, with intentionally left blank sections for the DM to input and create for his own devices. Today's adventure module is almost always going to have a story with a defined resolution. Some of these are contained with a single adventure, while others can span multiple different adventures across an entire region of said fantasy world, the adventure path. 

Companies like Wizards of the Coast and Paizo have made their bank accounts swell on the backs of numerous adventure paths over the years. Paizo in particular has a host of these which have helped shaped and define the setting for the Pathfinder RPG. Almost all of them start with the heroes dealing with a localized threat to a town or village, eventually expanding out to unravel in some cases a global threat which must be put down or doom to all who should live to see such days. I am fan of numerous of these adventure paths, having run and played in several and if you're curious, I previously published a ranking (at that time) of the 5E adventures which should be considered adventure paths for this discussion.

But what if I told you the greatest of all of these, was the very first one written primarily by the founder of the D&D game itself, Gary Gygax. Some call it the Greyhawk Adventure Path, the modules were all produced independently of each other, but weaved together they form perhaps the greatest D&D story and adventure ever told. For those unfamiliar these adventures are ordered as such: Temple of Elemental Evil (T1-4) >> Slavers series (A1-4) >> Against the Giants (G1-3) >> Descent into the Depths / Drow (D1-3) >> Queen of the Demon Web  Pits (Q1).

This series of adventures sees the lowly bunch of cutthroats and ne'ed-do-wells arrive in the Village of Hommlet looking for easy coin, but what they find is a dark cult thought put down, now on the fringes of escaping again. Surviving the epic Temple of Elemental Evil is no small task, but should they overcome, next they must unravel and get to the core of a rotten slave business plaguing the coast of the lands of Oerth, it won't be easy and they well find themselves slaves themselves if not careful. With renown such as they have earned by this point, only a rare few would be called upon to stop the march of the giants in the lands of Keoland. Is there anything that can stop these massive creatures and who is bringing them together on the small folk? Finally, the real threat is revealed, an ancient race who plagues the dark tunnels of the underworld beneath the surface of the world. Our heroes must pursue the culprits to their ancient and wicked city and stop the menace at its root.

This is the elevator pitch, but the plots interlaced into these modules is barely touched upon by Gygax, but with a bit of work and imagination, a good DM can easily expand the story that is occurring here which is epic in proportions. If you have not played or run this series, I can't recommend them enough, this is the original adventure path and in my opinion, the best.