btw x-2 last mission, since it's not really the most documented thing, is a standalone sequel to ffx-2 which is entirely about climbing a single tower. it's very mystery dungeon-like in premise and gameplay

Crypt of the Necrodancer

Tao's Adventure for the DS (think Azure Dream's spiritual successor, made by the same company).
E.G.G. (Elemental Gimmick Gear) for the Dreamcast mooostly takes place in one huge dungeon called Fogna, but there is kind of an extra dungeon on the side that you explore about 1/3 of the way into the game.

Maybe a stretch? but Void Stranger takes place entirely within one mega labyrinth, with all other locations being relegated to flashbacks, and you do come across fellow strangers journeying through the halls for one reason or another.

@KyuuMetis That sounds good enough I think

Shining in the Darkness

Going Under is technically a megadungeon, sort of. All of the dungeons are located below the hub building. It's a gray area, but I wouldn't be angry if someone considered it such.

idk if you’re counting dlc or not but i’d say fallout new vegas: dead money definitely qualifies

Crimson Shroud takes place in a single dungeon, the Sun-Gilt Palace of the Rahab.

elemental gimmick gear

i only see bangers

Ah yes, my least favourite genre

Some titles like Tower of Druaga and Return of Ishtar, Spelunker, Gauntlet 1 and 2, and Atlantis no Nazo might count.

Dark Spire and I believe Mon-Yu count in the "hang around town and ascend a tower" type of Wizardry-like dungeon crawler games.

@DoctorIssa waddles in front of your view and stops. A rumble is happening in my stomache, something is coming. I let out a nasty burp with terrible after taste that will stick with me for the rest of the day and makes me really not want to eat anything. Walks past a corner and you can no longer see me

(Just so you know i was not anywhere near you so that you could smell this or feel its heat, but I was close enough for you to see and hear)

can i get burped on too

@moschidae I walk back from the corner I walked past earlier to reveal myself before you standing on the sidewalk. I pause and take a breath. You see me rub my belly and let out a belch the like of which breaks everyone out of the stupor they have gained from years of living in the city. They all look like deers in head lights, scared of this new occurrence, while I looked thoroughly pleased. I must have ate something good. I then walk across the street and past another corner

I think Colossal Cave Adventure might fit here; the very first thing you do is enter the cave and it expands out into all kinds of labyrinths and interesting locales and vistas. There are dwarves and dragons and trolls and talking plants. It doesn't have a "boss" per se, and your goal going in is very much open ended exploration, not something like "climb to the top of the tower and kill the big bad" like in your description, so I'm not completely sure if it qualifies. Also great list!

@cowboyjosh that part with a goal was kind of just a guideline thing, exploring to explore is still dope.

i will never forget this moment

Eternal Quest (2004) is a mystery dungeon clone set across one dungeon, recettear is very much NOT across one dungeon in terms of gameplay, it's really just that one of the dungeons is hyped up more in the narrative