I found a website years ago that ranked scientists according to the estimated lives their inventions and discoveries saved or enabled. Since I've been unable to relocate it, here is my own list.
Agronomist Norman Borlaug is credited with saving a billion lives through his improved strains of cereal crops.
Karl Landsteiner gets credit for discovering blood types and thus enabling blood transfusions which have saved many millions of lives.
Fritz Haber of the Haber process is also near the top of the list as his invention allows for the industrial production of fertilizer.
For various discoveries and inventions involving alternating current and the electric motor, Nikola Tesla makes my list as well.
Edward Jenner and Jonas Salk deserve much credit for discovering vaccines and creating the polio vaccine respectively. Vaccination has saved the lives of more children than any other invention.
Galileo discoveries came at great personal cost as he was forced to under public humiliation and house arrest for saying the earth goes around the sun.
For many discoveries related to medicine as well as the food sterilization process which bears his name, Louis Pasteur earns a place on my list of great scientists.
And of course, the big three of Newton, Darwin, and Einstein were the most world-changing scientists.
I give honorable mentions to Leonardo da Vinci and Thomas Edison for being the all-time great inventors.