Why is 15 finger Sukuna the weakest?
Because :
Getojaku seems to be the main villain of the first part of JJK or one of the main villain alla round. He was leading the nature curse, was organazing the culling game, and he is 1000 years old and behind more or less everything happening now.
This will give him the first place here among them. And a place above a not at all full power Sukuna.
Yuta was tasked to defeat / keep in check 15 fingers sukuna, Yuta is the MC 2.0 and even has a movie for him, Rika is the queen of curse like Sukuna full power is the king of curse, Yuta was able to defeat 80 or 70% Geto during the prequel and now Yuta is even more experienced and powerful as he was able to train with Miguel, Gojo spoke quite highly of Yuta, and Yuta was said to be able to be as strong as Gojo in the future and was said recently to be second only to Gojo in unusual abilities.
So this place him above a Sukuna who is not at full power. Sukuna with 15 fingers is not at all at his peak.
And Hakari has his hype from Yuta statement that he may be stronger than him at his apex, and Hakari was said to be extremely strong.
For now no real feats but it's something.
So there is a case for Hakari > 15 Sukuna but for now i can place them = or 15 Sukuna a little bit higher