Best 2D game ever

In my opinion, Castlevania Symphony of the Night is the best 2D game you could ever play.
The storyline is pretty interesting for a game. You play Alucard, the half-vampire son of Dracula (don't ask me how a vampire could procreate, it is plot magic) who has entered the ancestral home of Dracula, Castlevania, in order to find out why Dracula's castle has reappeared. In the course of the game you meet Maria Renard who is trying to figure out what has happened to Richter Belmont, a vampire hunter. Depending on how fully you explore your environment and what kind of actions you take, you can unlock more areas of the castle to explore or get alternate endings to the game.
Castlevania Symphony of the Night is divergent from its predecessors because the main character can use many different weapons, and they are mostly edged weapons instead of whips. Also, the game advances non-linearly and you often have to go back over an area multiple times as you gain new abilities to explore with. Abilities you can gain include double-jump, transformations, and the use of familiars to help you explore. The sheer amount of items you can collect can keep you playing the game long after you have beaten it multiple times.
The main attractant of Castlevania SOTN for me, however, is the sheer beauty of the game. The artwork and soundtrack are neverending pleasures to experience. The backgrounds for the game are dynamic. Though they do not affect your character, there is action in the background of the rooms you explore. For example, in one room the weather outside changes, in another room there is a large eye that looks in on you from a window and follows your movements. The soundtrack is a diverse mix of jazz, techno, many genres of metal, and classical music, all gothic themed.
Though this game is sort of hard to find and relatively expensive (I bought my copy used at around $40), it is worth every penny. The replay value is immense as some features are only unlocked after you have beaten the game once, and you can always find a new item or easter egg in the game.