Castlevania: Symphony of the Night
Though I have played all of its spiritual successors, I have only just gotten around to the one that started the Metroidvania movement: Symphony of the Night. This was the first Castlevania designed by Koji Igarashi, now known as IGA, That Crazy Castlevania Guy.
Take the classical sidescrolling Castlevania formula, and add Metroid-style overworld navigation, equippable weapons and items, and all-around badass Alucard as your main character. Throw in a pinch of good musical composition and some awful voice overs and you've got yourself SotN.
It's clear what a giant leap forward this was in Castlevania history, but playing it now I can't help but recognize points of obsolescence. It is interesting to see features like the two-handed equipment system that have not recurred in the series, but it is also easy to see why, as it does not really contribute much to the game.
Progress: Level 16