Mine would have to be the one in the Royal Chapel. It's a stunning example of build-up while still keeping tension. You have this tremendously huge staircase where the rest of the structure's architecture depends on it with little crannies with stuffed goods every few steps. The colossal and strong diagonal shape is a simple but nevertheless powerful use of composition. The design itself also subverts Alucard's abilities with the Corner Knights, Bone Heads, and spiked balls, all placed on a sharply diagonal plane. The situation is unlike anything else in the game. I especially love the relationship between the boneheads and the balls. And to finish it off, the music with its airy choirs make the feeling of ascension all the more complete, which is all rather thematic for a "holy" place.