never crowned heir to the thrown: prince Albert Victor
Prince Albert Victor, Duke of Clarence and Avondale (1864-1892) was the eldest son of Albert the future king Edward VI, and the grandson of the reigning monarch, queen Victoria. Since Victoria's reign ended in 1901 he never was the official crown price. Albert Victor was known to his family as 'Eddy'. He fell ill with influenza in the great influenza pandemic of 1889–92. He developed pneumonia and died on 14 January 1892, less than a week after his 28th birthday.
On December 3rd, 1891 Albert Victor proposed to Mary of Teck. The wedding was set for February 27th, 1892. After his death she became engaged to Albert Victor's brother, the new heir, the future king George V. In 1893 they married and she became queen in 1911.