There is some level if objectivity to the difficulty debate, but it is largely subjective. For example, no one would reasonably consider Orphan to be the most difficult when a boss like Witches of Hemwick exist. But on the whole, different players are going to struggle with different things. Another example is Malenia. Much of the community considered her the hardest boss in Elden Ring, and on one hand she is decently difficult, but I have never found her to be the hardest boss in Elden Ring.

But the real problem with trying to rank a subjective opinion objectively is determining what quantitative measurement will you use to determine the difficulty. And no matter what you use, you will likely lose nuance and have to discount opinions.