The best of the 2D platformers. Period.
Symphony of the Night came at the tail end of the real 2D platformer generation, and that's easy to see in how much this game owes to its forerunners, not only Castlevania games, but also the classic names in platformers like Metroid. The graphics on this game, relative to other Playstation games and espeically to other games in the 2D platformer generation, are absolutely top-notch. The design team for Symphony of the Night seems to have gone all-out to create interesting locations, novel enemies, and entrancing special effects. Simple things like the blur-trail from Alucard's celerity lend a much greater depth to the story.
Gameplay is far beyond anything else I've seen offered in a 2D platformer. Alucard has hundreds of weapons to choose from, many with hidden special abilities that can have a powerful effect on the game. He has six familiars he can choose from, each familiar being able to level and provide new and original attacks. He gains levels, and has access to a store where he can buy yet more weapons, armor, etc. He obtains relics that give him new abilities. He casts a variety of powerful spells, if you stumble across the button combinations that cue them. There's hardly anything Alucard DOESN'T do in this game. With a number of possible endings depending on how much game completion you manage, there's incentive to discover everything you can. And when you're done with Alucard, you can go back and try it all again with Richter (a task not to be attempted by the faint of heart, though one with its own rewards in terms of cool new animations).
The only beef I have ever had with this game is the voice acting which, while often amusing, is so riddled with ham that it's a wonder the game doesn't have William Shatner playing a key role. Aside from the voice acting, I consider this a perfect game. Even with the voice acting, I consider this the best 2D platformer on the market and a game any discerning player should make an effort to get his or her hands on. You won't be disappointed.