Duke of Edinburgh
The title was first created in the 18th century, but merged back into the crown when its holder became king. It was revived several times, but became extinct on each occasion, as the holder died without male heirs.
Except in unlikely circumstances, the title can be expected to be merged in the Crown in the person of the present Prince of Wales. It was announced that, at that point, the title would be re-created for the Earl of Wessex.