It's probably the easiest Castlevania but this doesn't automatically mean it's a bad game. It's the best version to start in the series. III is already a tough one but compared to the Sharp X68000 version it's also just a walk in the park. I don't think that Akumajou Dracula on the Sharp X68000 is better just because it's harder. The arrange mode on the PSX lowers the difficulty to Dracula's Curse level (on normal). Better not talking about the later loops. Programmers had some serious fun with that one.
IMO there is no way around the NES originals if you really want to know what made people fall in love with Castlevania and why they (including myself) still hold the 2D games in high regard. The rather abstract, gothic, Western-style horror depicted in the NES games in combination with the vast technical experience and decidedly Japanese creativity of the late 80s/early 90s Konami developers led to games with very challenging but also highly rewarding gameplay that makes you come back for more even if it kicks your ass left and right.
What's more, the music of the NES games set the standard for things to come, some of the themes have become major classics of video game history and still sound as great and fresh as back in the day. Even if you don't pay much attention to it, the music compliments the visuals and gameplay perfectly and adds another dimension to the game.
Lastly, there is the overall level of difficulty and also frustration but I think Konami dosed that just right (well, with a few exceptions...), some parts are relentlessly difficult and make you tear your hair out but this makes it feel all the more rewarding if you finally manage to overcome them. In most cases, the developers found the magic sweet spot between challenge and frustration and gave players a task that seems overwhelming at first but is doable with enough practise and persistence, and you will gladly invest the time because it's just insanely fun to play.
So, my recommendation is to tackle the series in chronological order to really see what makes it tick. Specially if you love some of the later games that came out after the NES originals you just owe it to yourself to see what kicked off the craze and made Castlevania a household name among gamers worth their salt.