For protagonists: Hector.
Never mind that Curse of Darkness, Harmony of Dissonance and Aria of Sorrow (via the Castlevania Double Pack) were simultaneously my first ever Castlevanias. Hector actually gets a really good character arc (for an Iga game at any rate) about not letting selfish desires ruin you, letting go of past hurts, and moving on with your life. He's voiced by the superb Crispin Freeman during Freeman's golden period of voice acting, and Hector has a really cool general look. As the straight man to Isaac's crazy man, Hector is a great foil against his villain, and he got to have probably the greatest dialog interactions with Death in the series. Also his meeting with Saint Germaine is as silly and chuckle-worthy today as it was in 2005.
The only other Castlevania protags I hold with the same fondness are Soma and Shanoa, for the same Character Arc reasons. None of them are static protagonists. They all change, evolve, and grow over the course of their journeys, and emerge much different and improved people.
For villains? Walter. THAT. SEXY. ASS. VOICE.
Also he's probably the ultimate portrayal of a classic Fairy Tale villain in the series. But mostly that deep smexy baritone.