I absolutely LOVED Ultimecia's castle.

It was great the way they threw you in there and just said "go for it"

You don't have to beat all the bosses, and I thought that was great. I loved all the different sections of the castle, with my favorite being the Library. Vividarium et Intervigilium et Viator!

The foyer, the belltower, the library, the dungeon I loved everything about that place. The music was also incredible. My favorite final dungeon music besides Doom Castle in Mystic Quest.

Memoria and the Cleft of Dimension would be the runners up. Down at the bottom of my list would be Inside Sin and the Temple of Fiends.