In the other FromSoftware games I've played so far (DkS1, Sekiro, Elden Ring), the boss music themes were not really memorable, to be honest I forgot them shortly after the moment I beat the boss (and never felt like looking them up on youtube/elsewhere), can't remember a single one besides maybe Bell Gargoyles from DkS1. But with DeS it was different, I keep regularly coming back to its soundtrack. Vanguard demon from the tutorial, for instance - I tried to fight him but a couple times before giving up and proceeding to the main game, yet still I remember the soundtrack from that time, and listen to it every day.
Demon's Souls Soundtrack - "Vanguard"
Demon's Souls Soundtrack - "Phalanx"
Demon's Souls Soundtrack - "Armor Spider"
Demon's Souls Soundtrack - "Tower Knight/Penetrator"
Demon's Souls Soundtrack - "Maneater"