I'm a big fan of both, but I have to go with Super Mario World here. It was probably one of the best platform games to ever come out, and even to this day holds a feel of awesomeness that can't be beat. You can have your 3D platformers, I'll take SMW over any of them. Great levels, difficult levels, and great design all went into SMW, and come on, it's Mario 2D for fucks sake. Castlevania was a fun game and all, but it can't hold a candle to SMW.