Who was the first emperor of the Year of the Four Emperors in the Roman Empire?

The Year of the Four Emperors, 69 AD, was a year in the history of the Roman Empire in which four emperors ruled in succession: Galba, Otho, Vitellius, and Vespasian.

The suicide of the emperor Nero in 68 was followed by a brief period of civil war, the first Roman civil war since Mark Antony's death in 30 BC. Between June of 68 and December of 69 Galba, Otho, and Vitellius successively rose and fell before the July 69 accession of Vespasian, who founded the Flavian dynasty. The social, military and political upheavals of the period had Empire-wide repercussions, which included the outbreak of the Revolt of the Batavi.

Galba was the oldest emperor to date when he became Emperor and his physical weakness and general apathy led to his being dominated by favorites. Unable to gain popularity with the people or maintain the support of the Praetorian Guard, Galba was killed by Otho, who rebelled when Galba passed him over as his successor.