Christian VII of Denmark
Officially, Danish king Christian VII ruled from 1767 until his death in 1808, but for a large part of it, he was king in name only. Christian was considered incompetent not only due to his wild night life (he caroused with prostitutes in brothels) but also because of his mood swings, paranoia, hallucinations and self-mutilation. Some modern researchers have suggested that he had schizophrenia. Others that he had porphyria [sources: Rohl, Langen, Danish Royal Collection]. Ultimately he was mostly good for rubber stamping various decrees set forth by members of his court. He married the sister of King George III (yes, Mad King George), Princess Caroline Matilda, around the time he was crowned.
Christian's physician Johann Friedrich Struensee gained the confidence of the king and a lot of power. Christian gave him the title of State Councilor in 1768, and Struensee made numerous progressive reforms to modernize the country. That goodwill went away once Struensee began an affair with Caroline Matilda, and her divorce was finalized in 1772. Later that year, Struensee was executed [source: Toyne].
Both moves were orchestrated by Christian's power-hungry stepmother, dowager Queen Juliane Marie. She essentially ruled from 1772 until 1784, when Christian's son Prince Frederick VI took over as regent. Christian is rumored to have died of a heart attack or stroke after being frightened by the arrival of Spanish ships he thought were hostile. But there's not much proof to substantiate that [source: Schioldann].