King George III of England
By the time he died, King George III could neither see nor hear, and was considered completely insane. His urine was reportedly tinged blue and/or red, and stories had spread about crazy behavior such as attempting to shake hands with a tree because he thought it was the King of Prussia [source: Johnson].
King George III ruled from 1760 to 1820, and his other claim to fame apart from his madness, was that the American colonies were lost under his reign. He was also cultured and conscientious, and unlike many of the other kings on this list, devoted to his wife [source: The Royal Household].
Modern diagnoses of the cause of the king's insanity have included schizophrenia, bipolar disorder, sexual frustration or the hereditary blood disorder porphyria. Porphyria can mimic the symptoms of madness, causing confusion as well as red urine. Perhaps the arsenic in the medications given to him may have triggered or aggravated the disease [source: Johnson].
Scholars who believe that the king was truly mentally ill point to the disparate differences in his writing and behavior. In "manic" periods, for example, he had convulsions and wrote and talked excessively -- to the point that he foamed at the mouth. These scholars attribute his blue urine to the plant gentian, often used in medication [source: BBC].
In the last decade of King George's life, Britain was actually ruled by his son, the Prince of Wales, as regent [source: The Royal Household].