Probably Isaac Newton … he set the foundations for the way we think now. He came up with very very simple laws that can be combined together to explain very complicated things. He also invented calculus .. which may seem horrible at school – but it is used in literally every kind of science
Alexander Flemming changed life for ever when he discovered penicillin and used it in antibiotics – people now no longer die from tooth ache or a cut on the leg or an in-growing toenail. Penicillin totally transformed medical treatment.