Luffy has every reason to fuck around with Big Mom and Kaido. He has Future Sight, is very fast, crazy fast reflexes, is incredinbly flexible and agile, has insane physical strength and Advanced Armament haki ( Barrier + Internal destruction haki). The other supernova are not complete fighters like Luffy.
Luffy has all of his bases covered. Top Tier attack power, Top Tier Durability with Advanced Armament Haki Barrier, extreme reflexes and speed, Top Tier observation haki(Future Sight), Has flight with Gear 4. Lethality with Snakeman combined with Advanced Armament Internal Destruction haki.
Law is another person that can be confident, His devil fruit allows him to be.
Zoro has good attack power but average reflexes, bad observation haki, average endurance post timeskip, and poor mobility. Zoro is all attack power. 1 good stat does not help you win fights. It is a complete package that is needed.
Kid can create a railgun so he is good
Killer is hopelessly outmatched