People hatin' on Mt. Gagazet? Um, what? It has possibly the best area music in any FF game ever. That alone easily keeps it from being one of the worst locations.
I will confess to the fact I didn't like the Zanarkand Dome however. I appreciate they were going for a more intense feeling or something but after Zanarkand proper's amazingly somber atmosphere, it just felt disappointing.
I'm gonna also add both Sandseas from FFXII. Just awful.