Sadly, Marcus Aurelius broke the streak of Emperors selecting the most capable person as their successor, even if that man didn't happen to be their biological son. He appointed his son Commodus as Co-Emperor after his wife and best friend and champion general Avidius Cassius attempted a premature change of leadership while Marcus lay on what they thought was his death-bed.
Commodus became Co-Emperor with this father in 177 and reigned as sole Emperor after his father's death in 180 until his own assassination in 192. Not a bad length of reign for a lunatic.
He styled himself to be a reincarnation of Hercules himself. He was, by all accounts, a physically impressive specimen. He legitimately fought in gladiatorial combats and demonstrated his skill with weapons in sadistic shows in which he killed exotic animals such as elephants, giraffes, and ostriches.
Roman History is full of these colorful characters. He was strangled to death in a plot on December 31, 192.