Offensively? He beat down the East and West Coast Avengers, he's beaten the Fantastic Four + Original Guardians of the Galaxy, fought Keeper Surfer and the Guardians, you get the idea. He one shotted Collector BUT that version of the Collector was a geriatric with a heart condition (I'm not kidding, you can look it up yourself).
Defesively? He's hidden himself from the great powers of the universe : Watcher, Eternity, etc...
It should be noted that Kruggar (31st Century Sorcerer Supreme) 'one shotted' him by forcing him into a dimension where his powers didn't work or something.
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..even the outer hells are indifferent matters for they bow only to potent and archaic Nodens.
Starbreaker's feats sound similar -- what with ragdolling teams of very powerful characters and whathaveyou. As for energy absorption: Starbreaker was capable of draining Dharma... And Dharma was powerful enough to recreate the entire Milestone universe and merge it with the mainstream DCU... Without anyone knowing.
That said, Dharma is obviously far more powerful than Starbreaker. That feat just illustrates how unbounded his absorption abilities are.
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"I am tired of Earth. These people.
I am tired of being caught in the tangle of their lives."
Both are on the same level when it comes to energy absorption, although it could be argued that Starbreaker is more susceptible to energy siphoning. However, his susceptibility is counteracted by his ability to be empowered by negative emotional energy (which is what empowers him mostly). SB will empower himself with Korvac's negative emotional energy which will give him the edge to defeat him. That's how I see this going.
On the other hand, Korvac after his transcendence wasn't someone like Starbreaker who is out to kill, absorb, destroy etc. He was a man on a mission with a grand vision of a united universe with uplifted inhabitants. He genuinely regretted having to kill some of the Avengers, and in his dying moments he used the last of his power to revive them, even though they destroyed his plans. Okay some of the later writers somewhat messed the character up with lesser stories - but he wasn't really an evil, negative guy in the classic arc.
He doesn't have to be evil to feel negative emotions. The starbreaker was feeding on the negative emotions of the JLA, for instance. In the original Korvac saga, Korvac still felt emotions such as anger and rage:
which is what Starbreaker feeds upon. Also Starbreaker is not like the avengers or the GOTG (aka super heroes), so Korvac won't have any compassion for him like he did for them. And even then, by virtue of being the enemy, Michael will instantly have negative feelings towards him which will only empower starbreaker; I mean, remind me again what he did to the collector when he found out his plot against? it sure wasn't compassion.....and the same can be said about The Korvac Quest (which is the same version of Korvac). the only way to avoid this is if Michael actually knows SB's weakness - positive emotional energy -- which he won't.