Probably Sunless Citadel to be honest. It's light on plot anyway, but I've used the top layer of the citadel for a lot of different games.
I recently read through Sunless Citadel and I really enjoyed it. I plan on taking the bones of it for an adventure with my group. I like the overall concept. Large dungeon, two warring factions inside, each fighting each other for territory, and then secret dangers lying below.
I've got my party looking for several gems that will voltron into the main story macguffin, so I can easily just replace the family signet rings which are the default quest plot hook, with one of these gems.
I think I'm going to re-design the dungeon from the top down. I like the concept of the dragon cult temple that was swallowed by the earth. I might put it inside of a volcano or something. That sounds sick.
Then there are just a lot of weird old dungeon design philosophies that are present. Like empty rooms for the sake of filling out the dungeon perhaps? I've noticed this a lot in old adventures as I've been reading through old Dungeon magazines and modules. Sometimes as much as 25-50% of the rooms are nothing but dungeon dressing. I think part of this is to allow space to encounter wandering monsters, but it seems to be mostly ambiance and maybe tension? I don't know.
I want to rebuild the dungeon and make it more purposeful. Why would these rooms have existed in the first place, back when it was a cultist temple, and then what are they used for now that it's occupied by kobolds or goblins. I'd also like to try my hand at Jaquaysian dungeon design. It would make sense that the dungeon has more than one way in and out, considering these factions both exist there.
I also like how the original dungeon just has openings into the Underdark. Makes me wonder if this was inspiration for Baldur's Gate 3, where there is a tunnel into the Underdark hidden in the depths of an abandoned Temple to Selune, currently occupied by goblins. While this could be a great potential tie in for future endeavors, I'm not sure if I plan to include those or not though.
A session or two ago I gave my players a handout which was a found note, from a minor evil guy to a goblin minion. I have no idea where I'm placing this evil guy, but if I do end up running this adventure he can be the evil druid in the bottom of the dungeon.
All in all it looks like a stellar adventure and I can see why it has become so highly regardes.