Mine are:

1. A Little Knowlede, the pack-in adventure for the Dark Sun boxed set. I've used it many times, and it's always a hit. I do tweak it a bit to make it a bit more challenging. For instance, limiting the amount of resources the PCs find.
Tip: Waterskins are just as valuable, if not moreso than water itself. Being able to store and carry water is pretty important in a survival adventure.

2. White Plume Mountain. I don't care, I like the funhouse adventure. Erol Otus' map in the module is evocative and very cool. The question of what the players do with the three magic items always intrigued me, and my brother made a whole campaign around them, which we are currently playing.

3. I6 Ravenloft. Never played it, but I find the tarot reading and the randomized aspects of the module intriguing. I do hope to run it or play in it one day.

I never ran modules much before 5e; the few I read back when were not great, and so remained few. My experience with White Plume Mountain is from Tales from the Yawning Portal; it was very bad indeed. But I liked Forge of Fury from that book (played it and later ran it). Of the AL modules I've played or run, Outlaws of the Iron Road is probably my favorite; less railroad than most.

BX
Keep on the Borderlands: Still one of my go-to modules for starting adventuring and free roaming within a relatively small area.
Isle of Dread: My other go-to for wilderness exploration.
Master of the Desert Nomads: This and its followup module span a fairly good Arabian tales style adventure with lots of intrigue and plots.
The Lost City: This one I received as part of a batch of gaming material from a friends former DM who passed away. Its a really interesting adventure that takes things outside the usual.
A few others from that era.

AD&D
Expedition to the Barrier Peaks: This one is just so fun to spring on unsuspecting players.
Ghost Tower of Iverness: One of the first modules I ever bought. A pretty good puzzle adventure packed into a small area.
Dwellers in the Forbidden City: Got this ages ago at a con. Another fun exploration heavy adventure.

2e
The Darkness Gathering trilogy: This is hands down my favourite module set from 2e. It is a sprawling adventure that takes the players ever deeper into a possibly universe spanning threat. Together they form a large campaign.

5e
Tyranny of Dragons: These two modules are much maligned by some. But I had a blast DMing it. This is another sprawling adventure, a campaign really.
Descent into Avernus: Just got this and still reading through it, but several elements in it I am enjoying allready.
Tomb of Annihilation: Have this but have not finished reading through. Looks to be another good one.

For me it tends to be The New Hotness, but I do love Jean Wells' B3 Palace of the Silver Princess and hope to run it again soon.

Currently running Red Hand of Doom, definitely a classic adventure that lives up to the hype, players have frequently commented that it feels like being in a fantasy movie (it's closest to The Two Towers, if Rohan were Alabama).

Several of the Primeval Thule adventures I ran this year were fantastic; Night of the Yellow Moon and Watchers of Meng are probably the top ones from outside the campaign book, while Cavern of Golden Tears in the setting book is great too.

Quote from: Ratman_tf;11064512. White Plume Mountain. I don't care, I like the funhouse adventure. E

My then 9-year-old son absolutely loved White Plume Mountain when I ran it in my Classic D&D Karameikos campaign; the power weapons were needed to fight the upcoming war with the Master of the Desert Nomads.

S1: Tomb of Horrors!

It's iconic, its brutal and its so damn hard that players just yearn for the street cred of saying they beat Acerak.

BX:
The Lost City - It's a classic pulp sword & sorcery adventure and a good bridge from Basic to Expert levels. I feel like I need to run this one again soon.

AD&D:
The Sinister Secret of Saltmarsh - This one is just pure nostalgia for me.

5e:
Curse of Strahd - This is one of the more sandbox-y and least overwritten of the WotC adventures. And it's genuinely fun.

OSR:
The Tomb of Sigyfel - This is a very sparse intro dungeon for Labyrinth Lord. It shows you can do a lot with a little as a DM. It's a good intro to OSR-style play. Also really easy to re-skin into any campaign.

Maze of the Blue Medusa - I ran about half of this in 5e a couple years back. Great sandbox, very table-friendly and gives the DM lots of room for improvisation. For me the best part may have been crafting the setup to get the PCs to the maze itself.

OD&D: Caverns of Thracia by Paul Jaquays. This is still, IMO/IME, the best megadungeon and model for a megadungeon.

2E: The Shattered Circle by Bruce Cordell. This came late in 2E's run and is often overlooked but it's a very solid large dungeon with an interesting backstory that is easy to customise for a range of settings.

Old School:

-Keep on the Borderlands (D&D): A classic for a reason. No need to expand upon  this.

-Feast of Goblyns (AD&D2E): Lots of moving parts, memorable NPCs, and an interesting plot combined with plenty of "regular" D&D stuff. IMO the best of the 2e Ravenloft products.

-Mutiny on the Eleanor Moraes (Star Frontiers): Probably the best sci fi adventure I've yet come across. While on a survey mission on a feral alien planet, your vessel suddenly is forced to crash land due to sabotage, far from your base. The players must use their minimal survival equipment to get back to base, through many miles of hiking and white water rafting, and innumerable alien animals. Meanwhile the saboteur is back at home base alternating repairing the ship that is the only way off the planet, fortifying the perimeter, and sending security robots after the PCs, while hopped up on amphetamines. This was really ahead of its time and holds up well. The fantastic artwork is a bonus.

New School:

-Sailors on the Starless Sea (DCC): Enormously entertaining, small dungeon crawl. We ran this as a regular low level adventure, I'd love to run it as the 0 level funnel as intended. There is nothing really special about it, its just exceedingly well done. Really all of the DCC stuff by Harley Stroh is like that.

-Observer Effect (Delta Green): Challenging to GM with a large number of NPCs doing different things at different times in different places under different frames of mind, with (hopefully) minimal combat. This adventure succeeds at creating an increasing sense of panic and desperation in a way I haven't seen before.

-Curse of Strahd: Very well done for what it is. If you want to expand your proven adventure into a campaign this is the way to do it.

I dont use a lot of modules as a rule.  My favourite from the 2e days was the Night Below.

For B/X D&D: Good ol' Keep on the Borderlands. 32 pages packed with adventure and good basic DM'ing advice.
AD&D: Desert of Desolation series, GDQ series.
Warhammer Fantasy Role-Play: The Oldenhaller Contract, great beginning scenario. Any part of The Enemy Within, but for me especially Empire In Flames.
Call of Cthulhu: The Haunting, which has probably launched a thousand CoC games. Shadows of Yog-Sothoth, because Cthulhu is hungry and needs to eat those 1d6 investigators a round. Masks of Nyarlathotep, because if you don't stop ol' Ny, he's going to make sure the stars are right RIGHT NOW.
The One Ring: The Darkening of Mirkwood. Things get worse for Rhovanion, but in an epic way. Lets you interact with Tolkien elements as things build to the War of the Ring, and deserves to be mentioned in the same breath as other classics.

Quote from: Conanist;1106639-Sailors on the Starless Sea (DCC): Enormously entertaining, small dungeon crawl. We ran this as a regular low level adventure, I'd love to run it as the 0 level funnel as intended. There is nothing really special about it, its just exceedingly well done. Really all of the DCC stuff by Harley Stroh is like that.

Nice! I did run it as a funnel, and you'd be shocked (maybe not) at how many 0 level characters fell into the damn sinkhole on our first run.

I have fond memories of running the 2e Ravenloft Ship of Horror module back in high school. I may dig that up and see how I can work it into a Ghosts of Saltmarsh-based campaign.

For D&D variants:

I6 Ravenloft is an excellent module that nicely fits the genre and has story elements without being railroaded. It was followed up by I10 The House on Gryphon Hill (Ravenloft II) which has a more involved plot that is also non-railroaded.

C1 Hidden Shrine of Tamoachan is a little linear from being a tournament module, but it is full of unique flavor -- especially how it incorporates the player illustrations of most rooms. The fantasy Meso-American flavor is very well-done, and shows D&D branching out.

The Sunless Citadel for 3E is my favorite introductory module. There is a storyline to it with some meat, but simple enough for the PCs. It's got a nicely non-linear structure, with different branching while still making sense and having merges that bring things back together.

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For Champions, there are some great sourcebooks, but my favorite adventure module is The Coriolis Effect. It's nicely comic-booky in having a plot that encourages incorporating the PCs into it, rather than being a railroaded story or just villains to fight. It's a rare medium between an out-of-the-box adventure and a sourcebook. It has blocks, but also good discussion to direct the GM in creating something that fits the PCs more closely. Superhero games work better when there's some comic book drama, which the PCs should be a part of.

For Call of Cthulhu, there's The Golden Dawn, which is also a similar medium between a drop-in adventure and a sourcebook. It encourages PCs who are really a part of the story, rather than random elements that stumble onto an already-written plot. The flavor is also important - I thought it was excellent in merging Victorian mysticism, English mythology, and Lovecraftian horror.

For James Bond 007, there's Back of Beyond (second-billed as "You Only Live Twice II"). Most of the modules are poorly handled retreads of the movies, but a few tried to go beyond. This was an nicely original James Bond mission centered in Australia, which hits a lot of James-Bond-like tropes while also working as a challenging adventure.

My favorite RPG adventure/module ever is Masks of Nyarlathotep for Call of Cthulhu.

The longer answer:

My favorite D&D adventures of all time (roughly in order):

- G3 Hall of the Fire Giant King by Gary Gygax (D&D -  TSR)
- Maure Castle (Paizo)/WG5 Mordenkainen's Fantastic Adventure by Rob Kuntz (AD&D -  TSR)
- T1 Village of Hommlet by Gary Gygax (AD&D -  TSR)
- A1 Slave Pits of the Undercity by David Cook (AD&D -  TSR)
- Maze of Zayene #4 Eight Kings by Rob Kuntz (AD&D from Creations Unlimited, or d20 from Different Worlds)
- WG4 Forgotten Temple of Tharizdun by Gary Gygax (AD&D -  TSR)
- Dark Druids by Rob Kuntz (1e/Guy Fullerton's Chaotic Henchmen)
- D3 Vault of the Drow by Gary Gygax (AD&D -  TSR)
- Starstone by Paul Vernon (OD&D -  Northern Sages)
- S4 Lost Caverns of Tsojcanth by Gary Gygax (AD&D -  TSR)

The rest of my favorites are @ https://grodog.blogspot.com/2018/05/these-are-few-of-my-favorite-things.html

Allan.