A lot of great ones already mentioned in this thread. I'll add X2 1981 Castle Amber for Dungeon & Dragons Expert Set. I loved that adventure so much when I was younger.

My absolute favourite would be "Tegel Manor" by Judges Guild.

For a weird change of pace, my favourite would be "The Mansion of Mad Professor Ludlow" from Dragon #42.

There's a DCC module called Enter The Dagon... an intentional pun on 'Enter The Dragon'. It's about a tournament of wizards on a remote island. There is exploration and mystery and maybe a dungeon... but the tournament itself is the main concern and I found it very tense as things progressed.

The One Ring: I really enjoyed running (part of) the first adventure from the Bree-land book: Old Bones and Skin. Evocative stuff in there, very fun.

MCC: A Fallen Star for All and the one that came with the rulebook, forget the name.

DCC: My favourite modules of any system. Sailors on the Starless Sea and Doom of the Savage Kings being my two favourites. I've only run a small number of DCC modules though, sadly. They really have spoiled my appetite for more vanilla adventures. I'll start reading something like Red Hand of Doom (I started running that, only got to the lake up north before I became dissatisfied with the whole thing, and my gaming group), and although I see that it is solid for what it is, I grow a little bored of how neat everything is. Where's the weirdness? Everything is so... on-brand. The D&D brand that is. The black dragons are all poisonous, the red dragons breath fire, the hob-goblins are militant, the wizard in town sells potions, etc. etc.

5e: The Starter Set, the Phandelin one, was the first thing I GM'd, so I have a soft spot for it. I don't think I'd run it now though. Not unless someone paid me quite a bit.

Call of Cthulhu: I read the heck out of Horror on the Orient Express. I'd still love to try running that again. When I did we only made it two sessions in. Frickin' love the setting though, and the little vignette adventures interspersed throughout, flash-backs to the past. Perhaps too gory over all, in a way that becomes a little repetitive (how many times can the flayed skin animate and try to suck your fluids out through your stomach before you grow bemused with it all?), but man what an epic campaign. Has a little bit of everything.

Lamentations of the Flame Princess: I've never played LotFP, but I picked up the PDF of A Red and Pleasant Land recently, and although the edgier content has never been my bag, as I started reading it the creativity of the setting was really inspiring. Perhaps this is not technically a module though.

L1: The Secret of Bone Hill
One of the first true sandbox modules published and it still stands up well even today. It is packed full of content for such a small book. The adventure doesn't assume anything of the players, and for the most part they are left up to their own devices as determined by the rumors they discover. Running the module more than once will often result in a different campaign.

T1-4: Temple of Elemental Evil - I think it's a masterpiece, even if it is a hot mess.
I'm a little biased because I have a lot of memories with ToEE, both as a player and referee. I know the adventure inside out and that has certainly helped me to expand the setting and create the right mood for the adventure. Done right, ToEE makes for a great campaign full of lovecraftian horror with a dungeon containing distinct and apposing factions that can be played off one another.

Age of Worms (Chapter 1 and 2)
An interesting grim and urban sandbox set in an industrial mining town. Diamond Lake and its inhabitants are detailed to the point where there's a lot to do and see in the town alone. I'd recommend GMs be mindful of the events in chapter 2 from the onset, because that knowledge can deepen the experience due to some of the political elements that come into play. I wouldn't bother with the continuing story line however, because it takes you away from Diamond Lake and it has a distinct Paizo 'Adventure Path' feel after chapter 2. The dungeons are actually well designed as well.

Barrowmaze by Greg Gallespie
I think I would place it higher if it was not for certain editorial issues with the book. It's huge, and more importantly, it has a traditional theme. If you like gritty and gold for experience adventuring this is for you, especially if you want something of a larger scale than something like Keep on the Borderlands. You could easily use something like Fever Swamp and Dolmenwood alongside it as well, if indeed you wanted to expand the setting.

Okay. some old, some newer..

TSR - B1-B4

B1 In Search of the Unknown. 1977
Written by Mike Carr, this is still the perfect adventure for new D&D players, a descent into the ruins of an ancient keep in search of missing (NPC) heroes. The layout of the dungeons and caverns, as well as the open blocks for new GM's to add monsters and treasures ensure a high replayability factor. Clear the entire dungeon, six months goes by, new critters with treasures have taken up abode in the labyrinth of dungeon rooms and caverns. Originally released in the spring of 1977 with the J Eric Holmes Bluebook Basic D&D Boxed set.

B2 Keep on the Borderlands 1979
Gary's early design, based loosely on a combination of The Frontier Forts of Kelnore, and Blackmoor, This module for D&D beginners features a small hamlet with a Keep that the players can base out of, with the nearby Caves of Chaos populated by all manner of nasty critter that have been as of late, attacking the Hamlet and the Keep. An excellent beginners adventure that was originally released in 1979. Released with the later Moldvay/Cook edition basic D&D Boxed set.

B3 Palace of the Silver Princess 1981
Everything both right and wrong about D&D in one module. Recalled and destroyed due to objectionable art, only a rare few of the original still exist commanding premium five figure and up prices in collectors circles. This module features an evil Barony, and a ruined palace rumored to be the resting place of a fabled legendary Ruby. Notable for the large number of new monsters included with this module

B4 The Lost City 1982
Lost from a Caravan in a desert sandstorm the players run out of water and stumble into the ruin of an ancient city buried under the sands. Descending into an ancient stepped Pyramid the players find the first gateway to a subterranean city on the periphery of the Underdark. Notable for being an adventure that starts at for characters level one, but can easily be played and include adventurers of the 10th level and higher.

The Frontier Forts of Kelnore (Judges Guild) 1978
From Judges guild, this book provides a basic, modularized map of a border fort. All Kelnore forts were very similar border strongholds designed to support a small infantry and cavalry detachment.  Since their construction, the forts have fallen into ruin and many have been populated by various monsters. Tables are provided to put in random groups of creatures and individualize the forts, including treasure and features.  There are three sample forts already included, filled out according to die rolls on the tables, to illustrate their application. The built-in generators provide nearly an endless variety of strongholds in various conditions from just a pile of rubble, to a pristine brand new frontier fort. Great as an introduction to a wilderness campaign for a low level adventuring party after their first dungeon expedition.

The Thieves Fortress of Badabaskor (Judges Guild) 1978
The Thieves of Fortress Badabaskor is a scenario set in the five-level dungeon lair of a band of brigands. The upper levels of the dungeon are for low-level characters, but the deeper levels are more difficult. It includes a description of a new evil deity, Angall of the Perpetual Void. The Thieves of Fortress Badabaskor was written by Marc Summerlott, Bob Bledsaw, Mike Petrowsky, Craig Fogel, Bill Owen, and Tony Floren, and was published by Judges Guild in 1978 as a 32-page book. Notable because the first edition sold over 15,000 copies, and many more were printed, in several later editions. This also features a dungeon where low level players can begin, however difficulty rapidly escalates and this becomes more difficult the deeper one travels into the dungeons. Recommended is a party of 12th level or higher characters to complete this adventure.

Newer Modules...


The Forge of Fury (WOTC) 2000
Published with the release of 3rd edition, the Forge of Fury is a dungeon crawl, or site-based adventure, that describes the ruined stronghold of Khundrukar. The great dwarven smith, Durgeddin the Black, founded this secret stronghold within a great underground cavern system two hundred years ago when he and his clan were driven from their home by a horde of orcs and trolls. The orcs discovered the location of Khundrukar, Durgeddin's home, a century ago when they captured one of Durgeddin's clansmen. The orcs raised a great army that stormed the stronghold and slew the dwarves there, allowing the stronghold's five levels to fall into ruins. Now goblins, orcs and other monsters use the ruins as a base. Legends tell of the extraordinary blades Durgeddin forged in anger, enticing the player characters to come to the ruins of Khundrukar to obtain them. A tremendously good adventure suitable for new and veteran players playing low level characters.

The Bloody Sands of Sicaris (Paradigm Concepts, LLC.) 2001
This adventure features the party traveling to a wondrous desert city secretly hidden within a high mesa located deep in the desert in search of the missing daughter of an Imperial Noble. The balance of power in an ancient and venerable kingdom is shifting and the tides of history are surging around this ancient oasis in the desert. This was one of the first low/mid-level third edition adventures that focus on political intrigue, and is a masterpiece worthy to be included with any edition of D&D. Written and published by Henry Lopez, Pedro Barranechea, William Rivera, Eric Weiner and Nelson Rodriguez this was the Introduction for the world famous Arcanis campaign setting.

Kingmaker (Paizo Publishing) 2010
In 2010, Pathfinder's Kingmaker Adventure Path consisting of six modules, raised the bar on what a tabletop fantasy RPG campaign could be. With open-ended sandbox-style adventures, Kingmaker beckoned players to claim their throne and carve a new nation out of the treacherous River Kingdoms, map hex by map hex. Tens of thousands of gamers answered the call, ranking Kingmaker among the best-known and best-loved tabletop campaigns in history, and introducing a new generation to sandbox style wilderness exploration play.

Sailors of the Starless Sea (Goodman Games) 2012
A Dungeon Crawl Classics game, Since time immemorial, you and your people have toiled in the shadow of the cyclopean ruins. Of mysterious origins and the source of many superstitions, they have always been considered a secret best left unknown by the folk of your hamlet. But now something stirs beneath the crumbling blocks. Beastmen howl in the night and your fellow villagers are snatched from their beds. With no heroes to defend you, who will rise to stand against the encircling darkness. The secrets of Chaos are yours to unearth, but at what cost to sanity or soul?

In no particular order:
Return to the Tomb of Horrors
When a Star Falls
The Iron Orb of the Druegar
Wards of Witching Ways
Threshold of Evil
Hidden Caverns of Tsojocanth