Outside of Pompey, Caesar, and Scipio, if there is gonna be another Roman in the top 10 Western commanders it has to be Sulla. I'm actually not that familiar with Gustav at all, but I was curious. I'll focus my reply on Heraclius.
Heraclius campaign is certainly legendary, and stakes are absolutely insane. I still think it's a relatively short resume. He certainly uses the central position, defeat in detail, and guerrilla warfare to pitch perfection, but the enemies do a great job of giving a lot of room.
3 enemies armies, except the main force is just sitting there the entire campaign. Heraclius defeats the southern inexperienced army, then continues south pillaging everything, forcing the 2nd army to pursue out of anger and fear. Too quickly, the main force is once again uncoordinated. In haste, they fight another battle without coordinating with the main force. Then after the 2nd force is destroyed, the main force is destroyed in battle.
Great campaign, I wish his enemies were more coordinated in trying to destroy Heraclius. Obviously he deserves a ton of credit because his forces were outnumbered, mutinous and stuff. He also selected the perfect operational movements. But the enemy opponents did not do a good job at all in coordinating. So it's an all time legendary campaign, but weighed down by his opponents making uncoordinated movements. It's not his fault. He played his hand perfectly. But with how short the resume is and the points about the legendary campaign I pointed out, it becomes hard to rank him over someone with a LONG resume like Sulla, who also has moments of brilliance.
