I've never felt the love for the NES games of the franchise like a lot of fans did. Super Castlevania IV was the first one to caught my eye, making it some very interesting changes to the look and gameplay. The better graphics to the platforms and the ability to swing your whip was a very important upgrade to the fluidity of the game. Really diverse and engaging level design was the biggest strenght of the game, you could see how Konami took advantage of the 16-bit system.
I'm not the biggest Castlevania fan, but I would recommend to people that want to jump in, start with this one.