Symphony of the Night is the pinnacle of not only the Castlevania series, but of the whole 2D action genre, narrowly snatching the crown from Super Metroid, whose achievements it builds upon. It doesn't break any new ground, so to speak, but it takes the best parts of Super Metroid, the best parts of Castlevania, painstakingly polishes and glosses them up, then presents them flawlessly. This game is a combination of the best parts of the two best 2D action series - the result is as great as you'd expect.
The focus has been switched from the traditional Castlevania platform-jumping and monster-whipping to exploration and building statistics. In fact, the main character isn't a Belmont (though the Belmonts do figure in the game) - it's Alucard, the saturnine son of Dracula who takes up arms against his father. And man, is he cool. He may not have Samus's lithe body, but he has an assortment of swords and magical powers which accomplish many of the same things, and become just as nigh-invincible in the end. Like Super Metroid, Symphony of the Night is one huge, self-contained environment (Dracula's castle), but divided into unique parts that are intricately woven together. Unlike Super Metroid, Symphony of the Night boasts multiple endings and multiple modes of play (for instance, it is possible to play as Richter Belmont with the aid of a code). The game inherits and improves Super Metroid's attention to detail, enormous environments, emphasis on exploration, and moody atmosphere, appropriately Gothed up. There's also a great soundtrack (also Gothed up) and a protagonist (also Gothed up) cool enough to make many a pasty gamer vicariously live through him. There's even a decent storyline (albeit with bad voice acting). The result is the best damn 2D action adventure ever, and then some.