S Tier
Ebrietas, Daughter of the Cosmos- By far the hardest for me. It's hard enough before it gets to its second form if the camera screws up or you get trapped in a corner... and then the second form happens. Also frenzy lol.
A Tier
Vicar Amelia- I've heard this was easier with a heavy weapon, but it still gives me some trouble. One of the bosses where it's really easy to have everything go to shit really fast, and it has a ton of health.
Shadow of Yharnam- I thought this was really easy on my first playthrough... turns out that was glitched because the flame sword one wouldn't attack or call snakes. A lot harder the real way, even with parries.
Matyr Logarius- He can be really easy or hard depending on what he does, but he's more hard than not. I feel like this is one of the "did you get lucky?" bosses in the game. I used to think he was impossible, but he's not so bad now.
Gehrman, the First Hunter- I know I just said he was really easy if you parry him, and he is, but even then you can fuck up or get nervous and die really fast.
B Tier
Father Gascoigne- I swear no matter how many times I face this guy, I always lose at least twice. This shouldn't be this hard lol.
Moon Presence- Seemed a lot easier coming off of Gehrman, but it's still not too easy. Only fought this one once though, maybe I need to do it again.
Amygdala- Kind of borderline between B and C. Was really hard with my cane, but with the bloodtinge build I could just sit back and shoot really easily. Pretty middle-of-the-road.
C Tier
Mergo's Wetnurse- Not that hard to avoid outside of darkness mode, the hardest thing about this boss is how long it takes to kill.
Blood-Starved Beast- Another luck based boss I think. I've never been poisoned by it, but I feel like whether you win or not depends on if it tries that jumping grab attack.
Darkbeast Paarl- I have no clue how to beat this guy, but by the time I face him I'm high enough leveled to just use blood vials and tank his hits. He goes down fast.
D Tier
Rom, Vacuous Spider- This guy is too easy. The only time I had trouble with him was when I tried killing the spiders, as soon as I switched back to just running on my second playthrough I beat him lol.
Micolash, Host of the Nightmare- He's pretty easy (especially with blood rapture), but his Call from Beyond can be hard if you don't know what to do, and those dwarves suck. Probably my favorite boss fight.
The One Reborn- One-shotted this dude the first time, had a bit more trouble the second. You have time to change clothes and item equips lol.
E Tier
Celestial Emissary- I felt bad when I lost to this guy a few times in a row. Those mobs can get you by surprise.
Cleric Beast- One shotted on my very first time playing the game, never had any trouble since.
Witch of Hemwick
Witch of Hemwick- Yeahh...