Definetly Tony stark, shuri is smart but Tony invented time travel and a new element, shuri showed she was smarter than the avengers in infinity war, but remember that Tony was not with them at that moment. And shuri made all her Tech with many resources, Tony made the mark 1 in a cave from a bunch of scrap, while having to hide it from the terrorists who checked up on him every few hours.
For the strongest definitely scarlet witch, she destroyed an infinity stone while holding off thanos, who had 5 out of 6 stones, and still almost couldn't walk forward due to her power. The stones are said to be undestroyable, thanos has to snap the stones out of existence, and she destroyed the mind stone with one hand
