Thats quite hard to scale, cuz people can simply start to create situations that could've happened if a character hadn't done this or that. Opinions also can change depending on the point of view, especially because the arc isn't over yet.
- Luffy (i think it speaks for itself)
- Zoro (Due to sacrificing against "Hakkai" especifically, he basically sacrificed himself so the others could escape without thinking twice, without that, things could po8tentially get real ugly for Luffy, Law, Kidd and Killer. He also beat King, one of the biggest treats out there)
- Law (without a doubt, an amazing support and strategist on basically any situation, he saved Zoro from dying against Hakkai, Law was easily the most important on Big Mom's defeat, especially after his Awakening who turned him into one of the biggest damage dealers in Onigashima)
- Sanji (Saved Momonosuke, who is an absolute key character in this arc, saved lots of Samurai, Chopper and also Zoro in the live floor, blowing Perospero out of the Live Floor in the process, who was also a big threat arguably comparable to Jack, protected Chopper's medical squad against Queen while Zoro was getting treated, to the point he needed to hold King and Queen at once when Marco was out, King and Queen were already pissed with the situation and started going serious, something they didn't do against Marco for example. Sanji brought back the Alliance's hope at the Live Floor and also beat Queen, one of the biggest threats out there)
- Marco (Marco held out King and Queen for a good amount of time, but they weren't going all out against Marco at all, most likely the opposite, so he gets a point but he didn't do nothing that much relevant against K&Q, who just started getting serious when Sanji arrived there and Zoro got out of his bandages, although he was quite useful at being annoying, preventing a few Samurai from dying too fast due to Queen's plagues, and buying time overall until his stamina was out).
- Kid (Important on Big Mom's defeat, despite that he was basically carried on the rooftop and didn't do nothing relevant there, and he got kinda carried by Law against BM as well, the fact is that Kid and Law would get beaten by Big Mom anyway, just as they got more than once at that fight, the plot and the environment made Big Mom hold back and fall into their traps, we can acknowledge them, but the situations they were put in were extremely at the edge of everything going wrong)
- Yamato (She held Kaido for a bit, thats true, but we can't confirm that there wouldn't be anyone to make Kaido busy atleast for a bit, neither that Kaido would simply start a rampage when he hasn't even done that since the beginning, most Scabbards were already back on their feet and we still had Marco there. Kaido himself admitted he wasn't serious against Yamato but she was going all out, the fight itself was more about Kaido whopping Yamato's ass with ease while Oda took the time to introduce Yamato's backstory, Kaido wasn't focused on defeating Yamato but simply arguing with his daughter and "teaching her some lessons", until Luffy came back due to Momonosuke's help)
- Momonosuke (Has been important in many different ways, not only at giving the Alliance a reason to fight, but after he turned into adult he could help Luffy to get back to Onigashima quite quickly, and he is also holding Onigashima and preventing it from falling right away. He is also communicating with Zunisha and his role isn't even done, his importance could get even bigger)
- Inurashi/Nekomamushi (Beat Jack and Perospero, with extreme difficulty, which says a lot about Jack and Perospero's strength. Although we can argue that other Scabbards could defeat them if Inu and Neko were completely knocked out)
- Killer (He hasn't done nothing that much relevant overall, but he helped Kidd increase his perfomance against Big Mom by defeating Hawkins, who was nerfing Kid with his abilities, although we can't confirm that Kidd was absolutely nerfed and was going to die or something likely, because he kept fighting regardless of anything, Hawkins also wasn't that big of threat or an actual strong ass foe)
Honorable mentions (important on specific situations, but were left out in the grand scheme of things):
- Chopper, Robin, Jinbei, Franky, Shinobu, Brook.