Ego the Living Planet is one of those comic book deep cuts that no one really expects to see in any other medium but comics. However, he was brought into the consciousness of the greater public by the movie Guardians of the Galaxy Vol. II, albeit in a very different form than the one comic readers are used to.
The two versions are very different- the comic Ego has tremendous psionic powers, the ability to change its surface into whatever it wants, and create powerful bodies to use to battle its foes. Ego the Living Planet is a daunting threat for anyone, but how would he do against the villains of DC Comics? Which ones would he beat? Which ones would beat him?
10 Could Defeat: Reverse Flash
Reverse Flash is one of the most dangerous villains around. His mastery of the Negative Speed Force allows him to be more than a match for just about any being he goes up against, but all of that power won't help him against Ego.
Reverse Flash would be foolish enough to travel to Ego's surface and once he was there, he would be completely at the planet's mercy. Ego could overwhelm him with defenders or transform swathes of his surface into caustic substances or take away the atmosphere Reverse Flash is breathing.
9 Would Lose To: Mongul
Mongul is usually a guy who punches things until they fall. This strategy wouldn't really work against Ego, but that's not the only thing that Mongul could do to the Living Planet. See, Mongul is in charge of Warworld, a heavily armed planet that travels the spaceways and attacks whatever Mongul wants to.
Mongul definitely has access to weapons powerful enough to destroy a planet. The battle between Warworld and Ego would be epic, but Mongul could use Warworld's weapons systems to even the odds and hit Ego with an attack that would destroy him.
8 Could Defeat: Gorilla Grodd
Gorilla Grodd is hubristic enough to believe he could mentally dominate a being as powerful as Ego and so would attack the planet telepathically. This would be a huge mistake for Grodd because Ego is more than ready to deal with this kind of assault.
Grodd is very powerful and would be able to hold out for a while but Ego's overwhelming psi skill would take its toll. Grodd would disengage from the psychic battle, but Ego could send groups of his powerful creations after Grodd, overwhelming the gorilla.
7 Would Lose To: Brainiac
Brainiac would see Ego as the ultimate prize and take a lot of time planning out the perfect way to attack the Living Planet. He wouldn't rush things at all, sending clandestine robotic probes to the planet to see how it reacted to attack. After analyzing the info, he would begin his assault.
Ego is intelligent but Brainiac would make sure that Ego had no idea he was coming and have no time to figure out a way to deal with Brainiac's attack. The Living Planet's psi skills wouldn't help against Brainiac's robotic troops and Brainiac would implement his plan and take Ego out.
6 Could Defeat: Sinestro
Sinestro was once the best Green Lantern around and as such is used to dealing with all kinds of powerful cosmic threats, but Ego would be too much for him. Sinestro has always been very smart about how he uses his power ring but Ego presents a threat that would be too much for him.
The problem for Sinestro would be figuring out a way to defeat Ego before his ring ran out of charge and he'd fail at that. While he was recharging his ring, Ego would launch a powerful psi attack against Sinestro that would take him out.
5 Would Lose To: Perpetua
Perpetua created the first iteration of the Multiverse in the far distant past. Her powers are amazing and she can pretty much do anything. Ego is powerful, but he's just a planet with some psi powers that can make bodies and manipulate his surface. Against a being as powerful as Perpetua, there's pretty much nothing he can do.
All of his psi attacks would be for naught. He could throw legions of his created soldiers at her and she would uncreate them before they got anywhere near her. Nothing Ego could do would stop her. She could conquer his mind and make him fly himself into a star or blast him with enough force to obliterate him completely.
4 Could Defeat: General Zod
General Zod has all the power of Superman combined with the know how of one of the greatest Kryptonian soldiers of all time. Unfortunately for him, General Zod isn't really ready for a threat like Ego. His plans are typical to those of military men- use overwhelming force to defeat an enemy- and he just wouldn't be able to muster the kind of power needed to defeat Ego.
Zod is smart and powerful but he doesn't think at the scale that is needed to defeat Ego. He would go down to Ego's surface and try to fight the planet, but that would be a complete failure. Ego would overwhelm Zod mentally and destroy him.
3 Would Lose To: Anti-Monitor
Anti-Monitor is one of the greatest threats the heroes of the DC Universe have ever faced. His powers are based on anti-matter energy and he was responsible for the destruction of the original iteration of the DC Multiverse. He proved more than a match for just about any hero that he went up against. Ego is powerful but there's nothing he can do against the power of the Anti-Monitor.
The Anti-Monitor has destroyed entire universes full of superbeings. One planet with superpowers can't stand a chance against him.
2 Could Defeat: Lex Luthor
Lex Luthor is one of the most intelligent people on the planet. If anyone could figure out a way to take down a living planet, it would be Lex Luthor. However, Ego's psi powers are what would make the difference in this battle.
Ego could use his powers to read the mind of Lex and see what plans Lex is making to destroy him and then send wave after wave of his minions against Lex, overwhelming his security and destroying the lab. Lex couldn't resist forever and would fall.
1 Would Lose To: Darkseid
Darkseid is the God of Evil and leader of the planet Apokolips. He would go to war with Ego just because it was something to do. It would be an epic confrontation but not one that Ego could win.
The two would send legions of their lackeys at each other and Darkseid would actually be getting the worst of it. At that point, Darkseid would decide that fighting this way was futile and unleash the Omega Effect on Ego. It destroys whatever Darkseid needs it to- including a Living Planet.