I think George III has been rather unfairly treated by history. If you know anything about him at all, it's probably that he went 'mad', as portrayed in the film The Madness of King George.
It's true that he did have at least one major nervous breakdown, late in his life, which made him extremely ill. Whilst he may have had earlier bouts of mental illness, this last breakdown seems to have been triggered by the death of his youngest and favourite daughter, Amelia. The king never recovered.
Before his illness, however, he appears to have done his best to be a good king. He was a moral man and took his royal duties seriously. He married a princess chosen for him by his advisors, and though he didn't meet her until their wedding day, he and Queen Charlotte had a seemingly happy marriage, with 15 children!
He was the longest reigning monarch at the time - only Queen Victoria, and our current queen, Elizabeth II, have reigned longer. However, his reign was not especially peaceful since it included wars with France and the American Revolutionary Wars, which ended in America becoming an independent country.
When George finally became too ill to reign, his son, the Prince of Wales, who was also called George, became Regent (which is someone who rules in place of the king or queen while they are still alive). The Prince of Wales was Regent for the last nine years of George III's life. This period is known as Regency. When George III died, the Prince of Wales became King George IV.
George III, by Allan Ramsay. Fur was clearly in!