One of my favourite dungeons of all times was the adventure (which is 90% dungeon) Gates of Firestorm Peak. Here are some of the features that made me like it so much:
1: Not too big. It is a large dungeon (125 rooms!) but not quite a megadungeon. Near endless dungeons that never end are boring
2: Variety: The dungeon had roughly 4 "sections", each with different monsters, design and general feel
3: Made sense: the later end of the dungeon is fairly "gonzo" BUT the "nonsense" was explained, it made sense within the context - far realms contamination.
4: Factions: This dungeon had various groups inhabiting it, with different goals and motives. Some of these factions were hostile, some could be turned into allies etc.
5: a "tough entrance" - it explained why few outsiders were in there, and also made it difficult for the PCs to retreat - they had to survive in there for a while! I'll note that I don't find it fair to a party to "force" them into such a situation - they knew what they were getting into (sort of... but at least they knew to get provisions etc).
Another fun one was published in Dungeon Magazine, a 2e adventure by Perkins, called "My lady's mirror". This was a 70 ish room "dungeon" that was actually a castle, with several floors. The absent owner of the castle, a powerful mage, had trapped many foes in a magical mirror. The mirror was broken, releasing a flood of foes. This resulted in a very, very dynamic environment, with lots of RP possibilities, and provided creative PCs many options on how to enter the castle etc.