As most might already know, I get a lot of my design ideas from Everquest style dungeons, which I still consider as some of the best. The reason I think so, is they were “extensions” from the over-world, but a lot more difficult. You simply did not enter them, run through and finish them within 20 minutes. You often spent several levels in there and they were quite complex, making them hard to navigate. That is missing in modern MMOs in my opinion.
So with that being said, one of my favorites is still Guk in Everquest. It was multi-level having many areas above and below you, had two levels (live side & dead side) which satisfied MANY levels, Was quite large and complex to navigate, had numerous reasons to be in there with godly amounts of drops, and was extremely challenging because of mob density and mob AI. I miss the challenge those types of dungeons provided.