Either Augustus or Aurelian.
Augustus managed to turn Rome from a decaying Republic that had constant and devastating civil wars into the largest and most powerful empire of its time and his tenure as emperor created a remarkably stable system so that even when the empire was ruled by some of the worst rulers ever, such as Nero and Caligula, the empire managed to hold on. Also, the man was prone to sickness but that didn't stop him, he quite literally made his weak body obey his strong will numerous times, also also need to make a shotout to Agrippa, without him there wouldn't have been an Augustus, best bro in all of history.
As for Aurelian, while he only ruled for 5 years in those 5 years he managed to save the Empire from total collapse as it found itself in the worst part of the Crisis of the 3rd century, the Western and Eastern provinces had rebelled, large hordes of barbarians were running rampant through the border regions, disease was spreading, social cohesion was collapsing and the economy was imploding and yet this absolute gigachad of a man more or less fixed most of those problems in the 5 years he ruled as Emperor, reconquering the separatist provinces, defeating and sending the barbarians back into the shithole they came from (Aka G*rmany), upgrading the defenses of Rome, beginning the reforms of the army to make it more mobile and flexible, reforming the tax system and more.
So why did he got shanked? The answer is corruption, Aurelian had a massive hateboner for corrupt officials, handing out death sentences for relatively minor offences, eventually, a certain secretary knew that he'd be executed soon so he decided to forge Aurelian's signature in order to fake a document that said that Aurelian was going to go after the Praetorian Guard soon, the secretary passed the document to the Praetorian prefects and the Praetorians did what they did best and they killed the emperor, Aurelian was dead, the empire went back into crisis and things went back to shit.
Also Aurelian recieved one of, if not the most badass title in history, Restitutor Orbis, the Restorer of the World.