I like all the games mentioned above. But I gotta mention EverQuest as usual because it's so good. Or can be depending on how it's played.
It has the best dungeons I've ever seen by a long shot. Some outdoor, many indoor, some underwater. Many different themes and huge varieties of enemies and how they fight etc. Traps and secrets, items unique to only those places, etc.
The original game had lots of small to mid size dungeons but a few huge ones. Then the first expansions added more great huge dungeons. Some that come to mind. Velkator's Labyrinth, he's a mad wizard who lives at the top of an ice tower full of crystal golems and spiders and stuff. The floors are slippery. You work your way up a huge crystal spiral with battles on each floor. You can see another area but it is behind a huge wall of glass. At the top you have to find a hidden wall and then you fall down a hole and there's a village of Orcs (in the same dungeon). You go beyond them and there's a castle that was behind the glass, with golems and stuff. And at the top of that is Velkator. But there's also a whole lower level full of crystal spiders and kobalds and I spent lots of time down there ganking hoards of those kobalds for exp.
Sebilis - huge dungeon inhabited by Frogloks mostly but there's a huge dragon at the end of one wing. The left wing people called Disco, it leads to a Froglok Bonecaster who spawns on a platform that looks like a stage. It's a long way and a lot to fight along the way. The right wing goes down to some chef and dungeons and has a bunch of bosses. Then there's a central passage that requires someone to lockpick a door to a pyramid. Inside that are powerful undead and golems and some bosses. Then beyond that is a corridoor leading to a big boss in his own tomb and he has some special gear. Then there's a whole level underneath all of this with a massive dragon and a bunch of things guarding him.
Paineel/The Hole - I loved it because I had to spend a lot of time in there for an epic quest and I got to explore every bit. It's huge. You have to swim into a pond and lockpick a giant rock that is actually a door, just to get inside. Or if you can use the Shrink spell you can crouch through a gap. Then inside there are tunnels with stuff to fight, then a huge cavern with a bridge over it. Some people could spend weeks or months just fighting in that one cavern. But it's about the first 5% of the dungeon. Beyond the cavern is a castle underground which goes down deeper and deeper and is enormous. Multiple wings. One leads all the way back up to the top and comes out on that bridge. The other way leads down to a whole city underground. And beyond that it goes even deeper to another castle which is undead. And then beyond that is an underground raid boss called Yael. It takes a lot to reach him. Although sometimes you can jump off a cliff with levitate and land right next to Yael and save all that time. But it's not always possible to do it and survive.
Solusek B - it's a lava dungeon underground with rivers of lava everywhere you can fall in and burn to death in. It is mostly rock at first and lots of Kobolds to fight, a kind of base with 3 rooms with some boss kobolds in. Then there's a tunnel full of sonic bats and that splits. One way goes to Efreeti Lord who is a Djinn that is covered in flames and has flame imps to help. Tough fight. The other direction goes through tunnels with fire spiders and then there's a castle full of huge Fire Giants and a boss. Beyond that is the first raid encounter in the game, a dragon called Nagafen. That's all in one dungeon.
There are ones with evil eyes, underwater ones with sharks and mermaids and giant squids and stuff, a peaceful forest that gets overrun by undead at night time, a haunted house owned by a vampire, etc. It's really an amazing game. There is even one place that I am not sure was supposed to be a dungeon but a lot of people used it. It's a city called Highkeep and in the basement are enemies for newbies. But the guards who protect the city could be killed too and they get higher level as you go up the castle. Leading to a weird top level with corrupt guards, money being kept in a vault, a slave, an infiltrator etc. There was stuff going on, but you can kill em all for experience too. So that makes it a dungeon and I loved it because it was a cozy well lit indoor castle and there was a room that was easy to hide in.
The maps don't do it justice when you see how big it is in the game. But it might give an idea.