This is my ranking based purely on the lore of the bosses, I’m open to discuss. The explanation of the tiers:
Literal Satan - Bosses who are pure, irredeemable monsters that commited unimaginable atrocities
Pure Evil - Bosses who are still fully evil, but didn’t do that many fucked up things
Pretty Evil - Your average villains
Neutral/Monsters by nature - Bosses who are either fully neutral in terms of morality and/or aren’t sentient and are just of primitive nature you can’t really assign any morality to
Not that Evil - Bosses who aren’t of malicious nature and are just defending themselves from the player or have no evil intentions
Morally Good - Bosses who are pretty much the good guys in the story
Literal Saints - Bosses who are literal heroes
EDIT:
After reading some of the comments I definitely have to agree with certain points:
-I’d move Seath to Pure Evil
-Twin Princes and Gael to Morally Good
-Aldia to at least Neutral, but he’s probably near Pretty Evil
I didn’t expect for people to actually discuss this in depth, yall have some pretty good points ngl