Heraclius - YouTube
He enlarged the Empire and his reorganization of the government and military were great successes. His attempts at religious harmony failed, but he succeeded in returning the True Cross, one of the holiest Christian relics, to Jerusalem. He also introduced Greek for official use by the State, which replaced Latin.
Is he anyone else's favourite Byzantine Emperor?
I consider his era the most interesting of all the Emperors. IMO the "Roman Empire" was lost under his reign - but it wasn't his fault.
The war with Parthia had been handed to him by the 700 year long history of Rome's counterproductive and unnecessary wars with the Persians (in general) and by wacko general/Emperor Phocas' inept rule (in particular.)
It's hard to say if Heraclius ever had any other alternatives than to war with Khosrau II, but that war was one war too many for the Romans.
When Muhammad came marching north, the Persian Empire was completely broken and the Byzantine Empire was only in marginally better shape.
At the time, Heraclius was attempting to regain control of territory which had been under Persian rule for a decade and a half. There were many problems in the Levant and Egypt, meanwhile he still had the Turks threatening Constantinople from the north. At the same time, all the loans to fight the Persians were being called in by the Church, while also at the same time, he was obligated to pay his army off and allow much of it to retire - as far as they were concerned the campaign was over.
It was touch and go anyway, but the straw which probably broke the camel's back was his failed religious policies. He had clamped down on the Ghassanids because of their orientation towards Arianism and had sent their King into exile - his payback came at Yarmuk when the Ghassanids switched sides on the day of the battle and the Muslims prevailed.
The accord he had made with Miaphysitism was far too little and far too late. The Nestorians in the Middle east were much more comfortable under Persian rule and many of them jumped to the side of the Muslims at the first opportunity. But again, this can't all be laid at Heraclius's feet, the Chalcedonian persecution of Nestorians had been going on for hundreds of years at that time.
So, as good a general as Heraclius was, he was carrying too much baggage from the past to face the challenges that faced him.
Too bad so sad.