Aesthetically, my favorite dungeons in HW have to be both Gubal Libraries; the environments have an interesting feel to them and are detailed in the right ways if you bother to look around. They both start off nearly identical (granted, it's the same location), but I like how the atmosphere in the HM version let's you know something's not quite right (complete with the slow beginning of the new remixed track). I also like how the two dungeons seemed to reverse their progression of atmosphere; whereas the normal mode you travel from something more seemingly normal to sinister/neglected in the basement, hard mode has you travel from something sinister (or ominous) to a more well-lit/maintained section on an upper floor.

My aesthetically least favorite dungeon is Hullbreaker HM, no matter how anyone justifies it to me, it feels like the least-inspired revisited dungeon with the excuse that it was turned into a training-ground area to do as little with it as possible and call it "new." All the barricades, gates, bridges and structures ruin a lot of the original beauty of the normal-mode area, and this dungeon makes the "hallway with mobs" feeling stick out to me more than any other dungeon did in HW.