I'll just give a short rundown of my favourites and the ones I hated.
<spoiler=Abyss Watchers>Holy. Fuck. This boss is my absolute favourite. The lore, the music, the fight itself... It's just so good. First off, lets just say that it's one of the most noble Lords of Cinder. They knew what was the abyss and the danger it posed, so they banded together in undeath and fought it, every sign of it, even razing a country to stop it, fighting like and army of brothers. As such, they linked the fire and consequently, linked themselves together, sharing a soul. Awesomest boss in the game.
<spoiler=Dancer of the Boreal Valley>Awesome design in almost every aspect. Music, movement and visuals. Her unnatural movements are very disconcerting to the point of being hypnotizing, something like a snake slythering across the castle foyer, with nothing but her eerie music punctuated by her footsteps. Also, has the best ass of all the bosses.
<spoiler=Lorian, Elder Prince>Okay, this boss is kinda unfair playing solo, but me, a dedicated sunbro, fought him in jolly cooperation both in my world and others, so eventually I ended up liking him. His brother, Lothric, I liked because of his attitude towards the Linking of the Fire. If you played Dark Souls 2, then you know that the Linking is useless and does nothing to alleviate the entropy consuming the world, neglecting his duty as a Lord of Cinder. Hopelessness is one of the elements that permeate the franchise, so having a boss that recognises this is a positive mark for me.
<spoiler=The Final Boss>I loved the fight with the Soul of Cinder, not because of its difficulty, but more because of its lore. You're essentally fighting everyone who has linked the fire in history, including Gwyn, the Chosen Undead and the Bearer of the Curse. A love letter to end the franchise, it was both melancholy and very very cool.
<spoiler=High Lord Wolnir>One of the easiest bosses, I beat him very easly in both NG and NG+, to the point that he didn't kill me once, but barely managed to hit me in both. His lore is kind of cool, I guess, but still just way too easy for a giant boss anyway.
<spoiler=Deacons of the abyss>THE easiest boss by and large. Again, I never died, not on NG or NG+. Since I was very determined to not spoil the game, I only knew their name, so I entered with a fellow sunbro and proceded to absolutely wipe the floor with their collective fat asses. With my second go, I just herded them with the Coiled Sword weapon art. Boring.
<spoiler=Crystal Sage> Managed to kill me a few times, mostly because of his stupid doubles spamming me with their fucking Crystal Soul Spears. Killed him without much joy, as he can be absolutely be destroyed by pretty much anything if it weren't for his irritating teleport spam.