Ikana Canyon, all the way. The entire place has one of the most unnerving atmospheres in gaming - decrepit and yet functional. It's empty, but you're not alone. Running through with a Garo mask on (^^) only makes things creepier.
On top of that, it's one of the most extensive and challenging pre-dungeon quests in any Zelda game. First one must obtain the Captain's Hat from Skull Keta, then use it to break into a grave and learn the Song of Storms. Then one must play it to restore the river, and thus enter the music house and acquire the Gibdo Mask. Using that mask, one must traverse the Bottom of the Well, which - without prior knowledge - can be a very difficult task indeed. Following that, one is led right into the Ancient Castle of Ikana, which again, without preparation can put an intense time strain on players, and learn the Elegy of Emptiness. The Elegy must then be used to scale the Stone Tower, and thus enter the temple. PHEW! That's a lot.
In the midst of that, the area gains a lovely irony - the music box house's wonderfully cheery music in such a dark and haunted place gives me chills just thinking about it.