I stand by Simon's Quest as being a very innovative game for its time. It was one of a few titles that basically helped to pioneer the open-world genre, and unlike The Legend of Zelda, which was built ground up to be that way, Simon's Quest was an example of things that could be done with Castlevania besides pure platforming. That being said, I do love me some Castlevania 1 & 3 and Super Castlevania IV!
I still say one of the coolest moments in gaming history was the inverted castle! With the internet still in its infancy at the time, it was awesome hearing from friends that you needed the holy glasses to see what was controlling Richter to unlock basically a whole second half of the game. SOTN stands as my favorite PS1 game, I can't say enough good things about it!