In terms of disappointing it's probably Symphony of Sorcery.
Granted I don't remember much of Fantasia, but the world was quite boring and the scenery underwhelming. The music and the flow of battle were like a huge dissonance and didn't fit together at all. A lot of it was sickly sweet rainbows and clouds everywhere and it was a pretty boring place. The only two areas I liked were the place Sora finds his Sound Idea on the hill and the field of glowing flowers in the night you can see with Riku, but you never even go there.
La Cité des Cloches comes close solely for the fact that there is no Hellfire battle theme.
Symphony of Sorcery is actually my favorite ever, I have never loved the scenery of anything like I did that world. Even YMX stops to say just how beautiful the world is. And the classical music, especially on Riku's side was simple profound.
For me? The Keyblade Graveyard/Badlands, for a final world that just won't do. It's plain and is just one path really. I wanted to actually go through the fields of endless keyblades, climb mountain tops and actually work my way to the final battles and have cutscenes showing TAVs reaction to the whole thing as they travel the land.
"Follow the hearts and you'll find the way...that's something Master Aqua taught me once."--King Mickey.