I only own the first two, and I find that the second installment, Simon's Quest on the NES, is the best game in the series, making it into my top 25 games I've ever played. The combination of platforming, sidescrolling, adventure, puzzle, and RPG elements are stellar. It's a lot of fun when you collect hearts to buy the Flame Whip, find all the hidden roads and false floors, find the use of all of Dracula's body parts, and explore the immense overworld. The only other game that implements these features (to my knowledge) is Zelda 2, and that game in my opinion is better than Simon's Quest, but I digress.
The AVGN did in fact point out a number of reasonable flaws, such as the useless clues given by townsfolk, stiff controls, and hard to make jumps. However, cryptic things like the crystals aren't downfalls, they're just more reason to explore the game. I actually noticed a pattern; whenever you happen to get a new crystal in a town and go further onwards, in the event that you would hit an unpassable road-block, I've noticed that equipping the new crystal and ducking solves the dilemma.
Also, the music. THE MUSIC. Hearing Bloody Tears for the first time hooked me instantly. Yeah, yeah, I know I said how great the platforming and stuff was great, BUT THE MUSIC. IT IS AWESOME. Seriously, if you thought Megaman 2's Dr. Wily theme or Super Mario RPG's Geno theme would haunt you till the rest of your days, you have yet to experience the wrath of Castlevania 2's soundtrack. Brentalfloss actually wrote some good lyrics for it (despite it's strong language), and I would check it out.