I've played:
Castlevania, Vampire Killer (the MSX2 version of Castlevania), Simon's Quest, Dracula's Curse (the Japanese version, titled 悪魔城伝説 (Akumajou Densetsu) has a better soundtrack thanks to the additional sound channels of the VRC6), Super Castlevania IV, Rondo of Blood, Bloodlines, Akumajou Dracula X68000 (including its PSX remake, titled Castlevania Chronicles), Dracula X (SNES), SotN, Castlevania: The Adventure, Belmont's Revenge and Castlevania: The Adventure ReBirth.
Sorted in order of preference:
Super Castlevania IV > Rondo of Blood > Dracula's Curse > SotN > Akumajou Dracula X68000 > Belmont's Revenge (amazing soundtrack for a GB game, IMO) > Castlevania: The Adventure ReBirth > Bloodlines > Castlevania > Dracula X (SNES) > Vampire Killer > Simon's Quest >> Castlevania: The Adventure (the soundtrack is quite good for a GB game, though).
As for Simon's Quest being second-to-last in my list, it's not bad, it's average, in my opinion. The only bad Castlevania game in my list is Castlevania: The Adventure, mainly because of the
horrible jumping physics.
And now for several Castlevania songs which I believe are underrated:
Aquarius from Akumajou Densetsu
Ripe Seeds from Belmont's Revenge
Death Fair from Castlevania: The Adventure
Load BGM from Akumajou Dracula X68000
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