With a lot of publishers and their incoming games, it took a few years to truly develop something of substance. The early Famicom games would be mostly glorified golden-era arcade games, but after the release of Super Mario Bros. the system would seemingly be bursting to the brim with platformers and probably the earliest and most original after Mario, was Simon Belmont in his first adventure for the system, Castlevania.
I think the coolest thing about Castlevania itself is just the premise. You're hunting down Dracula, taking down other classic monsters in the process. The game is colorful, but still creepy. The controls are perfect for what they are trying to do, regardless of how much people complain about not being able to jump in mid-air. There isn't much story, but how many games in this period has a fleshed out story. The sound is fantastic. Castlevania as a series is notorious for its music and it all started with this game. The theme for the first stage is one of the most popular video game songs ever made.
Castlevania puts together a love of classic horror, great visuals, fantastic music and great control and comes out as one of the strongest NES games ever made, even later on in the NES' lifecycle. Castlevania is a true classic in every sense of the word.