chevkoch: Castlevania: Symphony of the Night. If you have not played it, I implore you to check it out. It's an Action/Adventure platformer with RPG elements, tons of weapons and secrets to stumble over, a huge list of enemies and magic attacks, lots of gorgeously crafted 2D environments, one of the most atmospheric soundtracks in gaming, in short — and I've said this before on the forums — it's a masterpiece.
I'd recommend playing the original PlayStation version (because it includes the few FMV sequences), but the Xbox Live Arcade incarnation is true to the original apart from the missing video, if you wanna go that route.
And don't be deterred by the RPG aspects mentioned, these are RPG-light and won't require much micro-managing — the game is at its heart a Hack 'n Slash platformer.
PS review Xbox Live Arcade review EDIT: Ah, and I already have Dungeon Keeper.