Sir Frederick Grant Banting was the key scientist who discovered insulin, revolutionizing diabetes treatment worldwide. Before insulin, diabetes was a deadly disease with no effective treatment. Banting, along with Charles Best and John Macleod, conducted experiments in the early 1920s that led to the isolation of insulin from the pancreas.

Banting had the original idea to extract insulin to control blood sugar levels. Charles Best assisted him as a medical student, while John Macleod provided the lab and guidance. Leonard Thompson was the first patient to receive insulin therapy, marking a new era in medicine.

Their discovery made it possible for millions of people with diabetes to live healthier, longer lives. Banting and Macleod were awarded the Nobel Prize in Physiology or Medicine in 1923 for this breakthrough. Banting shared his prize money with Best, recognizing his important role.

Here’s why other options are less accurate:
- Charles Herbert Best played a crucial role but was not the lead discoverer.
- John Macleod was the supervisor who helped in the research.
- Leonard Thompson was the first patient treated, not the discoverer.

In summary, insulin’s discovery is credited primarily to **Sir Frederick Grant Banting**, making him a hero in medical history and diabetes care.