Ultimecia's Castle is the only one that I can really vividly picture. This is probably because it was the one I've played the most, but I do really think it's the best out of the ones I've played. I love the castle design. The music is nice and fitting. The battles are fun. The puzzles are pretty enjoyable. I didn't as a kid, but I do now like the concept of unlocking your commands in there. It made it so you can control the pacing a bit if you want and skip some of the boss battles once you've got the specific commands you want unlocked. Everything leads to an actually pretty intense and fun final battle.

Usually, dungeons don't really stick out much to me, but this one just always has.