Memoria (IX) is my favorite.

Next comes the Interdimentional Rift (V), especially the Final Floors.

Sylx Tower and Dark World (III) come third, and boy, does Crystal Tower (like it's called in DS, I believe) look impressive in 3D! You got to love that music for Crystal Tower (DS), I find it absolutely awesome. (Heard it in a video, if it really was the music of CT.)

But yes, I do found Ultimecia's castle quite good, but I never was able to solve that **** riddle with the organ, which anoyed me more than a bit. Oh well, then I fought Ultimecia without Draw, like it was important, anyway. But the music of Ultimecia's castle I found really good. And the design of the 'dungeon' was good as well. I found it actually original that a plain normal castle was the Final Dungeon.

Also, all the music of these dungeons is great. Dark World especially if you consider the console it was made on.