Demon’s Souls gets pretty creative with its bosses compared to the games that came after it, both in terms of gameplay and aesthetics, so here’s how I think they rank from best to worst based on various criteria:
Old Monk
Storm King
Adjudicator
Flamelurker
Dragon God
Maiden Astraea
Old Hero
Fool’s Idol
Old King Allant
Phalanx
Tower Knight
Penetrator
Armor Spider
Maneater
Dirty Colossus
Leechmonger
Vanguard
And thus was the world mended.