My real favourite is Castlevania 64, but my secret favourite is Harmony of Dissonance. While it sort of copies SotN a lot, willingly or unwillingly it ended up being a very unique entry in the series for its eerie atmosphere, crazy colour palette and strange soundtrack. It feels like a scary nightmare stuck in time in the normal castle, then shifts to a drug-trip nightmare once you enter the mirror castle.
If you are stuck and pissed off just look up a guide online, I think it's worth playing to the end, there a few hidden things and a fun boss rush mode, not to mention other replay modes.
Also, I keep on losing the link to a forum post (on a different forum) of a guy who made a room-by-room analysis of the game. It's the best Castlevania article I've ever read, but you must like the game. Someone has the link?