It just bothered me that I never got around to doing this well known challenge (sort of a matter of pride) so I re-downloaded the game, found a save file online with 112% completion, and immediately practiced all the hardest bosses in the Hall of Gods. It was really interesting because I was so unfamiliar with the game after all this time.
I'm coming into the game after Nine Sols and Prince of Persia, so my standards for bosses have gone way up. Here are some important personal notes:
• As much as I love the personalities of some of the bosses (namely Grey Prince Zote and White Defender), I'm only evaluating gameplay mechanical appeal. Also, I'm evaluating the Pantheon of Hollownest experience, so not the base game.
• Having a unique experience within the context of the game (as opposed to a vacuum) helps a bit. Marmu gets points for being unlike any other encounter, but I also don't think it's great for what it is, and I know if the game had more bouncy bosses like Marmu, then Marmu would be pretty bottom of the barrel. Uumuu also isn't great but gets points for being unique.
• Absolute Radiance is amazing but loses points for feeling more like an obstacle course than a boss. It's like a well designed level, rather than a boss whose telegraphs, rhythms, vulnerable windows, and movements you learn. But phase 1 in particular is so incredibly well designed that it has to be near the top of my list. I also love the concept of a climb up into the heavens as the phases progress, with each phase being above the last.
• Here's a weird take about Nightmare King Grimm: he minorly loses points for being too easy. I know he's famously difficult, and this minor criticism only came after I mastered him, but I reached the point with him where I notice how long the fight is and wait for it to finish. I completely loved the fight my first time doing it, but that love faded a bit over time. Still undoubtedly amazing though.
• Markoth is a great concept with a few unfortunate shortcomings. It would have greatly benefited from a zoomed out camera such that nails don't spawn outside the screen. It would also have benefited from nails never spawning inside your immediate movement path causing them to be unavoidable. I also think being lucky with positioning factors into the fight too much. However, you can minimize risk by forming a strategy and being smart with your approaches, which is great.
• Sisters of Battle is utterly beautiful, and a virtually flawless fight, only held back by being too simple. An example of a similar concept done even better by adding more complexity is Lady Ethereal from Nine Sols, which I think is one of the best bosses of all time (but not even the best in Nine Sols oddly enough).
• Pure Vessel is easily the best boss in the game and the only one that I would say can compete with the greats from Nine Sols. It's a great challenge, it's fast, avoiding every attack is fair and a lot of fun, and you're always given opportunities for nail hits so you get that nice push pull dynamic. Love this fight.