Hidden Town for me - I like how there are three things in there that can be acquired, not just one or two. Also, I like the town location.
Spell spell - IMO, the most useful/helpful magic in the Valley of Death, with or (especially) without the Candle & Cross. Magic Key - too bad it came late in the game and not even really necessary to complete the 6th temple. Final Magic container.
Jump, Fairy, Reflect, and Thunder are all needed to beat the game. Spell is needed to beat it unelss you want to repeatedly grind for potions. Shield and Life are very useful spells. But Fire is a pretty much useless spell, plus there's nothing else special about Nabooru, plus you can get from there to Mido (and vice-versa) with only one or two battles.
So the town Nabooru is pretty much useless, and you lose very little if you completely bypass it.