I think I am going to be boring and list a theory not a device. The double-blind randomised placebo controlled clinical trial. This is the ONLY way to tell whether a medical treatment works or not and has revolutionised medicine more than any X-ray machine or MRI scanner. It has saved countless lives and prevented many toxic drugs getting on the market. Until its invention (late 19th century) doctors in ALL countries killed more people than they cured. Now it is the opposite.
And the greatest exponent of evidence-based medicine. Florence Nightingale. For some reason she is remembered as a lady with a lamp being nice to patients. in fact she was a gifted mathematician who analysed her hospital scientifically and revolutionised hospital services. But because she was a woman she is portrayed as just a “touchy feely” nurse rather than the great scientist she really was.