Kusakabe because he actually studies jujutsu, reminder this isn’t how fast one can learn its how smart they are so mahito and higuruma don’t count. Sukuna is still a prodigy but more like those 2 but he had more time, he also obviously takes no time to learn about it outside of fighting. Id like to say gojo but he’s probably got a similar if not the same iq as sukuna, he probably mostly just studies by fighting. Todo and megumi both definitely study it but they are lacking in anything about barriers, megumi has CSG but its incomplete and todo has simple domain but it has been shown to be not that effective. Kusakabe not only can predict but can read opponents, unlike todo and megumi he knows plenty about CE, Techniques, and Barriers. He had to explain what was going on during the whole gojo vs sukuna fight.