By platform:
NES: Akumajo Densetsu (the japanese version of Dracula's Curse). It has a great OST with the exclusive chip it has in this version, alongside having a more balance difficulty, and not changing stuff like Grant being able to throw his knives. It has the great mechanic of having multiple characters (that the Bloodstained: Curse of the Moon subseries also has it), and multiple stages. A truly delightful experience;
SNES: Super Castlevania IV. A great reimagination of Simon's first adventure, with new gameplay improvements like being able to swing the whip in 8 directions, grab some places to overcome hurdles, and using the Mode 7 for great visuals;
PSOne: Symphony of the Night. Not much to say aside being my first entry in the series, and being in love with the RPG elements, OST, a second castle to explore, and the cheesy dialogue;
Game Boy: Belmont's Revenge. A vast improvement in comparison to the weak first game on the portable. Better OST, gameplay and challenge;
Game Boy Advance: Aria of Sorrow. Consolidated the metroidvanias here. Better graphics, Michiru Yamane returns as a composer, nice gameplay system with the souls;
DS: Dawn of Sorrow. Even though some touch screen effects can get in the way, I had a blast with this entry, almost as close I had with SOTN. Having more than one inventory, and the extra mode making you able to level up as well made it a great entry.
Special mentions: Bloodlines (Genesis), Chronicles (Sharp X68000/PSOne), Adventure Rebirth (Wii), Rondo of Blood (PC Engine), Curse of Darkness (PS2/Xbox)