My favorite dungeon is...a dungeon. I need a complex hewn out of the living rock, worked by tools or creatures with natural carving powers. It needs to look organized from above, laid out with purpose - even if the purpose is to kill the invaders. I guess I have OCD when it comes to dungeon design.
Now don't get me wrong, I have nothing against organic adventure sites. D1-D2-D3 rank right up there on my all-time favorites list, especially when you throw in the fact they started...wait for it...under a dungeon. There is just something extra-special about going into a fortress, wizard's tower, etc. and discovering a hidden dungeon underneath. Cliche, perhaps, but I like what I like.