By my own feel, the top five for difficulty: Erlang, Yellow Loong, Yin Tiger, the Great Sage's Broken Shell and the Yaksha King.
Let me start with Kimimaro the Yaksha King. He is the only one on this list where I part ways with a lot of people (forum posts and Hupu ratings alike). I don't know why; I just found him pretty hard. His attack range, his aggression and his numbers are all fierce. Could it be because I used Spell Binder? Spell Binder is honestly a bit addictive: once you've used it, you want to use it on everything.
Erlang: I beat him with Spell Binder too. Later I saw people say there are lots of builds for him, even wheelchair-tier builds that carry you, but I never read guides for fights. Going in blind, I found that Erlang knocked me straight out of my transformations, Immobilize sometimes got bounced back, and several attempts died halfway when my mana ran dry. So my personal best solution (my only solution...) became Spell Binder, and at least I got to experience the game's top boss, built out of piles of money, fully and purely through the controls. At nearly level 100, I squeezed every point out of the skill tree. For spells I took only Spell Binder; for stances only Smash Stance (Resolute Strike works well in phase one, and used well it breaks his poise fast). My stamina was plenty, so I pulled every point from stamina recovery and stamina saving and put all the rest into base stats: attack, defense, health, crit. For armor I also went for the highest base stats I had, which I recall was half the Bull King set and half the Monkey King set. The staff was the Staff of Blazing Karma (not much use, since Erlang absorbs its fire). Curios: one for lightning resistance, plus the Gold Sun Crow and Jade Moon Rabbit pair for attack. In the gourd: Dry Spirit, soaked with Iron Pellet and a lightning-resistance soak. This may be the only build in the whole game I can remember, and it was all paid for in tears. Worth mentioning: I beat Erlang, enjoyed the giant-form showdown to the full, and just as Erlang opened his third eye in the cutscene, the game crashed... It was probably the only crash in my entire playthrough, and it happened right then. I nearly went red with rage and was already thinking about uninstalling. In the end, restarting just meant enjoying the giant-form showdown all over again... Game Science, you rascal...
Yellow Loong: to some extent, his numbers are just absurd. I hadn't started using Spell Binder yet and had brought Guangmou without much thought. That dash of his, with slashes that switch between fast and delayed timing, is hard to dodge and hits hard, so I felt I had no choice but to bring Cloud Step (and even then, because I hadn't maxed its duration, I had to dodge the last hit every time).
Yin Tiger: the earliest of the top five in the story, and probably the boss that feels best to trade blows with. When he swings that huge ship-cleaving blade, he looks impossibly cool. His grab with no wind-up can be answered once you've tried it a few times, but when I met him my level wasn't high enough and I didn't even have Spell Binder yet, so he schooled me for a long time. A master of rhythm, fierce yet slow and graceful. One of his moves triggers slow motion, and if you dodge it, the moment plays out like a film. By the way, he is also my favorite of the ten transformations (sorry, Guangzhi).
The Great Sage's Broken Shell: the story's final boss. By then I was wearing the Monkey King set, the Great Sage's own armor, so how could I change into anything else? The set adds damage after you cast a spell, and besides, fighting the Great Sage means trading spells with him; that is the romance of it. So no Spell Binder. In phase one I worked out how to cheese him with fourth-level charged heavy attacks (just remember to dodge the spinning flying staff), and phase two was all flash; the Plantain Fan is still great. While everything is on cooldown, all you can do is pray, but charging heavy attacks to break his poise felt like the right direction: the Great Sage isn't very aggressive, and his poise is low. Also, when the Great Sage steals my liquor, it doesn't heal him. He really let me off easy. Thanks.
Beyond my top five, a few others I thought were strong. The three chapter-end bosses Yellow Wind Sage, Yellowbrow and the Hundred-Eyed Daoist Master: one has a grab that could save the Chinese national football team, one is a mechanics boss, and the other is a mechanics boss too (a control type, sticky and clingy, a bit like fighting his junior sister and her lover). Cyan Loong is also quite a stat check, just a notch below Yellow Loong. Captain Wise-Voice is a disco, a bit like a bullet-hell game; I forget which move was the nastiest, and only remember the fight being a bit boring. Non-Able: what, he's only a chief? I'm not able to believe that.
Then the other way round: two bosses everyone calls hard. I nearly beat Tiger Vanguard on my first try and got him on my second; the Scorpionlord took about four. The Supreme Inspector I seem to have beaten on my first try (give me back Fourth Sister!)... I can only put it down to this really being an ARPG rather than a pure action game; the difference in feel probably comes from how far along your build is. Oh, and what's a pill?
To see the whole from a small part: I have rambled for quite a while and mentioned only a dozen or so of the 81 chiefs and kings, which shows how much the game holds. Across all its parts, I'd call it two parts rough, five parts on par with the mainstream and by the book, three parts pleasant surprise, and behind it all you can see a hundred parts sincerity and passion. This is China's first AAA game. It earns the title, absolutely.
Douban: Black Myth: Wukong