I think it's not easy to choose.
Castlevania IV introuduces Death with impeccable timing, after fighting Slogra and Gaibon, he's the last entity protecting Dracula. He arrives as if a last-defence trap has been sprung, and is summoned from another dimension. A genuinely eerie sprite with a gorgeous translucent, otherworldly effect. That last blow dealt on him feels so satisfying.
But then there's Rondo's ghost ship level which is a series highlight, and while it may not seem a natural habitat for Death, it works well, especially when he jumps down onto the mast with you.
I'll also mention the original Castlevania: where he was pretty tough if you weren't properly prepared.