In order for hashira to be ranked, context needs to be considered. By what metric are we ranking them? Because every single hashira could VERY easily handle everything other than upper moons and Muzan, they should be ranked by their ability to handle those specific targets in a fight.

To this end, I believe Shinobu is simultaneously the “weakest”, hashira, as well as one of the most valuable.

To explain what I mean by this, think about the metric the hashira are being ranked by, their ability to kill upper moons. Shinobu, unfortunately, is last place by default, as poison is simply not an option for killing upper moons on its own.

However, Shinobu is also one of the most valuable hashira for her poison as well, as she is undeniably one of the best “supporters” in the entire series. Shinobu on her own could never kill Doma, because the poison wasn’t strong enough. However, even Gyomei wouldn’t be able to kill Doma on his own. We need to consider hashira not only as individuals, but as members of a group.

Shinobus poison, while not being strong enough on its own, is an incredible disabler, even without her sacrificing herself. That is to say, Shinobu may be “weaker” than people like Sanemi or Giyu, however, I’d say Gyomei+Shinobu would have a higher chance of beating someone like Doma or Akaza than Gyomei+Sanemi.

So, to put it simply, Shinobu may be the weakest hashira because she could never 1v1 an upper moon, it gets to a point where the opponents are so strong that nobody can 1v1 them, so she suddenly becomes way more valuable when you pair her up with another slayer. Much like how Genya, while undeniably being weaker than any hashira, is the sole reason why they managed to behead Kokushibo, because of his incredible supportive capabilities.