Here's how I would probably rank the bosses in terms of difficulty myself*, from S being hardest to E being easiest:
Tier S
- Orphan of Kos
- Laurence
- Ludwig
- Amygdala (Defiled chalice version)
- Watch Dog (Defiled chalice version)
- Abhorrent Beast
- Bloodletting Beast (Depth 5 chalice/Headless version)
Tier A
- Loran Darkbeast **
- Lady Maria
- Martyr Logarius
- Ebrietas (Chalice version)
- Undead Giant (Depth 3 chalice version)
- Pthumerian Descendant (Depth 5 chalice version)
- Keeper of the Old Lords (Depth 5 chalice version)
Tier B
-
- Ebrietas (regular version)
- Rom (chalice version)
- Bloodletting Beast (Depth 3 chalice version)
- Blood-Starved Beast (Depth 4 chalice version)
Tier C
- Father Gascoigne
- Vicar Amelia
- Blood-Starved Beast
- Darkbeast Paarl
- Yharnam Queen
- Rom (regular version)
- Living Failures
Tier D
- The Shadows of Yharnam
- Cleric Beast
- Amygdala (regular version)
- Moon Presence
- Pthumerian Elder
- Beast-Possessed Soul
- Micolash (only here because of a potential OHKO attack, he'd otherwise be in the lowest tier
)
Tier E
- Celestial Emissary
- The Witch of Hemwick
- The One Reborn
- Mergo's Wet Nurse
- Merciless Watchers
- Watch Dog of the Old Lords (depth 1 version)
- Forgotten Madman
Tier "why is this even a boss":
- Loran Silverbeast
- Maneater Boar
- Brainsucker
Mentioning some chalice versions whenever relevant, sometimes there's a huge discrepancy between versions (Rom, Watch Dog, Amygdala), sometimes minor but still there (Bloodletting Beast), sometimes irrelevant (Merciless Watchers are always pretty easy for instance).
* Speaking generally, 'cause a lot of the time it depends on your build. For instance I struggled against Lady Maria the first time with my str build, using a kirkhammer (transformed, which was a mistake -- got her far more easily once I used the non-transformed one, and on a dex build I wreck her), and defeated the Orphan and Laurence on my second try, but overall I consider them harder than Lady Maria.
Same with Father G; he's the non-optional story boss that killed me the most in my first playthrough, but now that I am better at parrying and know about the tiny music box (I missed it first time) I consider him a lower-tier difficulty.
** Been a while since I bothered clearing the Lower Loran dungeon though, but I remember him being pretty hard.
I'm sure plenty of people would strongly disagree with my tiers though.
Especially where I put regular versions of Rom and Blood-Starved Beast, and the Shadows of Yharnam, which were never an issue for me.