Best in the series, with more variety and better control response.
Still, there is a lot of additional variety. You're still fighting demons, aiming for the big baddie himself, Dracula. Along the way you will be able to choose you path and depending on which path you pick the game will be easier or much harder. You can only meet certain characters on certain paths, and these characters may join you on your quest. You can switch to the alternate character at any time, and each character (including the hero, Simon) has their own unique abilities. The thief can climb walls, the wizard has powerful magic, the vampire can fly after turning into a bat (there are three alternate characters in total). You have secondary weapons/magic that add some spice to the game. However, you can get through every level and complete the game without any special abilities.
The first couple Castlevanias had some control issues. The stick just was not that responsive. Here the controls are much improved, and you can get very precise controls of the characters.
Graphics: At the time the graphics were pretty good. Today they are muted, of course, but at the time they were good. If Ninja Gaiden was a 10 this would be an 8.
Sound: Pretty crappy sound effects, but so was everything on the NES. The music was good, though. Moody without being repetitive or overbearing.
Gameplay: This game is just fun, and the variety of characters and paths adds tremendous replayability.
If you've never played Castlevania before, this is a good game to start with. If you have but have never played this version, get it, it's a classic.