Probably the first one. I played way more of the second one as a kid, but I have grown to love the original the best. The third one was cool and offered good ideas at the time, but I guess I gravitate towards a more simplistic style when it comes to the classicvania games.
I dig Castlevania III and all, but the first one wins, possibly by virtue of how often it's been drilled into my memory over the years. Even CV3 has a level where you basically are back in the first level from 1.
The first one was my favorite. I loved the challenge and amazing music. What's sad is I beat it in a weekend as a kid when I rented it, but couldn't the last few times I've tried.
I have longer and fonder memories of Simon's Quest and Dracula's Curse. And I think I appreciated Simon's Quest the most. Love the non-linearity. Too bad it was so cryptic and broken. Pretty sure I never got anywhere in it as a child because of that garbage. I'm tempted to play the re-translation someone did. I think there's even a hack that fixes the night/day mechanic
But I did so love crawling around on the ceiling as the thief in Dracula's Curse
Ohhhhh yeah, I remember hearing about that patch for CV2. I never got around to trying it, but I found it on Romhacking.net and both reviews are pretty positive, so I'll have to play through it later. Like you said, the original was a bit too cryptic, so this will likely make it much more enjoyable. It is a bit sad nothing was done to the bosses, though -- being able to walk past them kind of deadens the experience.
IMO the 3rd one expanded upon what the first one introduced to us. The 2nd, I remember only as something an old buddy's brother had. I remember he had a password for the flame whip, which was pretty awesome @ the time.
The only one I owned on the NES was Dracula's Curse. I got to Dracula a couple of time, but I think I beat the blue demon form only once. I got the first on the GBA when they did the Classic NES series. I enjoyed it a lot, but couldn't get past Frankenstein.