I adore the original castlevania; probably one of my favorite games. As for my absolute favorite, I'd go with Dawn of Souls; random drop farming aside, it basically improved everything I loved with Aria of Sorrow (a game I've played so much, I practically know the layout of Dracula's Castle). I've played a little of Order of Ecclesia, and I think that one's pretty kickass- it has the feel of metroidvania, but the difficulties of one of the classic games.