Mine has to be Inner Horutoto Ruins. Windurst players need to go down there for the missions "Lost for Words" (level ~20) and "Written in the Stars" (level ~30), and the only ways to get to the pertinent parts are past aggressive level 90+ monsters.
I mean, sure, you could buy sneak and invisible from the tome at the entrance, but everybody knows you can't open a door while invisible, and guess where these new monsters are lurking!
I'm glad that Square-Enix is working hard to keep those alchemists in the black.