I think Rappan Athuk has to set the curve. First, for its scale. It may have rivals in terms of total room count (though I wouldn't bet on that), but if you count 'quality' entries, rooms and other locales where some thought was put into who or what is there and what is going on, it is simply tremendously big. Second, it has a fantastic sense of large-scale geometry and integration. It is a spider's web of entrances, levels and sub levels, and connections. I think there is something like a dozen major ways in and out, and there must be a hundred paths reaching across levels. Imagine an intricate, maze-like 3D-map of rooms in a normal scale dungeon, and then imagine each room on that map is a level. Third, it has a very large number of interesting monsters, traps and potential foes and allies. There are too many to go through in a post, but the author takes familiar ideas (vampire, giant octopus, etc.) and makes them fresh. 'Factions' are often cited as a key element of megadungeons; I'm not sure I agree; they are just one color on the pallet and can be good, bad or irrelevant. Rappan Athuk has several organized groups you could think of as factions if you like, though I don't think of them as being the main point of the place.