I regularly whistle Vordt's fight theme while walking to work. Getting weird looks sometimes, it's a bit too epic for casual whistling.
Of note also, Flamelurker's theme, Maiden Astrea, Old King Allant and One Who Craves Souls. Holy shit Maidean Astrea's theme and its seven-note loop over a four-note rhythm. Demon's Souls OST was way more experimental than the others.
Edit: also The Nexus and its three second silences because fuck you