Here's my personal difficulty ranking for a lot of the named bosses (leaving out the invaders/crypt bosses etc because I don't remember WTF they were called):
Radahn (probably)
Messmer
Commander Gaius
Midra
Finger Mother
Putrescent Knight
Bayle
Scadutree Avatar
Leda battle royale
Dancing Lion
Romina
Golden Hippo
Rellana
Having done one full run as a caster and almost done with the second as a tanky bunga, some of those are way off for me.
Radahn - No question, every sin this game commits is in this fight. Shield and Spear basically required here, unless you are an Asian OCD click god.
Leda battle royale - Largely depends on how much you fucked up quests and how big the enemy team is. Can be easy with a tanky build.
Midra - Total frustrating pain in the ass.
Bayle- Frustrating, but manageable. Almost easy with shield/bleed counterpoke.
Messmer - Dies fast to bleed. Shield users have it easier. Casters trying to not summon or ash are in for a rough time.
Commander Gaius- I cant say a guy I one shotted both times is hard, but the concencus seems to be that he is.
Finger Mother - Trivialized if you have any ranged bleed, like Rivers of Blood will put him away easy. The Silver Mirror Shield negates most of his attacks. Complete puss.
Putrescent Knight - Again, build dependant, but a good shield and holy attacks (or golden order buff against undead) shred him. Uzi Spear plus any sort of Ash is cake.
Scadutree Avatar - A lot easier once you know the fight. Bring some good holy resist and fire damage and its a lot easier of a fight. This is an endurace test, so alpha strike builds have it rough here.
Priest guy with the Uzi Spear before Nightmare Woods- If you have a physical weapon art that can either peg him when he is behind the adds or along with his adds (Ruins GS, Rykards, Seluri Spear, Hora Luis's Earth Shaker, ect) then you can just burn him down rather easily. If you don't, this one is rough. As a caster, Ancient Rancor Call and Night Comet worked for me here. I don't know what the fuck my dagger person is going to do in this fight.....
Golden Hippo- Some people get hung up on this guy, and his hitboxes are bullshit (especially even the nerfed grab), but he is weak to lightning an in a room full of water, so......
Dancing Lion- Entirely build dependant and how many shards you have, but if you can cope with the spammy AE crap (either use an ash or keep close with shield) he is pretty much just an Erd Tree worm thing all over again.
Cerrilean Coast Death Bird- Complete joke if you have a golden order weapon or spell to merc the adds and him. Cluster fuck if you don't, but still managable.
Romina- I assume this is the dancer? Ancient Rancor call and Ruins GS weapon art flattened her so fast I barely saw ahat she did, so no real opinion here. But if no one ever summons me to help with her, she can't be too tough. If this is the scorpion centaur bitch, still easy and almost any tactic works here as she is basically just a Maleketh fight in most ways.
Rellana - Initially when adjusting to the new style of gameplay, she was rough. Once you figure out that you can jump the big tripple AE and that she (like hippo) is both weak to Lightning and mostly sitting in the water, she is pretty easy to beat. If you can't beat her any other way, Silver Shield stuffs most of her bullshit.
Cerrilean Coast Tibia Guy- Don't let the discus guy add and again golden order weapon just melts him and his adds.