Caverns of Tracia, Dark Tower, Dungeon of the Mad Mage, Expedition to Castle Greyhawk, Halls of Adren Vul, oh my

dark tower, mad mage, delving through the thracian caves. castle greyhawk, arden vul, castle of mad archmage too. dwimmer mount, barrow maze, chaos way down in the caves. lonely mountain, stone hell, tonigsborg dungeon as well, WE DIDN'T START THE FIRE-
- IT WAS ALWAYS BURNING AS THE DICE KEEP TURNING

Megadungeons can be great, but they're way harder to run because you're working with a very limited palette as a DM.
It's very easy to get a bunch of content for minimal effort by relying on traps and combat encounters, but these get boring for your players pretty quickly.
As long as you can offer enough variety with your encounters, along with rewarding your players for remembering details from past trips in the dungeon they can be a blast.
- Factions. Have different similar groups of enemies/npcs (or potential allies) that are fighting with one another. Have your players throw you for a loop as they scheme and become double agents who end up murdering both factions.

My players dipped into one of the mega dungeons I've made
Shit was wild and they plan to go back to explore some more but they're wanting to get stronger first
Little do they know there's MORE mega dungeons to be found that they haven't discovered yet
Edit: The one that they have been into had different layers with different kinds of environments and critters. They only made it a few layers down before they turned back due to the dangers.

One of the Bioware Neverwinter Nights expansions was to go inside some mega dungeon that I know existed in the lore from reading the FR DM guide with all the history and such; but I can't remember what original module it was taken from. IIRC, the dungeon was made by Elminster? I just remember recognizing the name and then being kinda disappointed with it because in NWN it's not quite as big as I had imagined. lol
- Undermountain / Halaster