Symphony of the Night still feels like a Castlevania "best of". Rondo of Blood, Super Castlevania IV too.
I'll post my top-5 from the DS games.
Order of Ecclesia: Mechanical Tower/ Styx Passage
(I loved some of the backgrounds)
Order of Ecclesia: Ecclesia
(not the best music track of the game, but lovely backgrounds)
Portrait of Ruin: Dark Academy
(this felt so right for a stage where the boss is Frankenstein's creature, great music too)
Portrait of Ruin: City of Haze/ 13th Street
(essentially the same stage, and I loved both BGMs used, the new one and the classic Iron Blue Intention)
Dawn of Sorrow: Condemned Tower (great music, and also loved the boss. For some reason that big "dinosaur", Gergoth, despite his size, his HP that didn't seem to end, and the swearwords I said until I finally beat him, he looked more pitiful than menacing - I think it was supposed to be some creature that was crippled by the one that captured and imprisoned it)