Going to have to go with Ultimecia's Castle as well. The design of this entire place was just incredible.

I can't really think of any point to criticize it. The place is massive, but FFVIII lets you explore freely with Enc-None. The visuals are fantastic. Overall design and layout is quite excellent. The puzzles range from kind of derpy to rather deep, and they're all fun. The mechanics of having to unlock your abilities is a brilliant twist, and it is made even more awesome by the gaming making all the fights optional. If you don't want to unlock everything, you don't have to. You can just unlock what you want and go. But if you want to explore this dungeon to its fullest, then you can spend a lot of time facing off the myriad bosses.

Brilliant final dungeon design.