Thanos is one of the strongest beings in Marvel comics and the entire MCU. He is so strong that it took several Marvel heroes to sacrifice their own lives for his defeat. When being placed into the DC Universe amongst more equal competition, Thanos would have a much more difficult time winning a battle.
The DC Universe houses supervillains that take on superheroes such as Superman and Wonder Woman (who are immortal life forms themselves). Likewise, these supervillains have to be equally as strong or vastly superior. As a result, this makes for some of the strongest beings in any Universe that could either take on Thanos or get completely obliterated by him.
10 CAN DEFEAT: Ra's Al Ghul
One of the main villains that is seen fighting the beloved DC hero Batman may be a challenge for the Dark Knight but wouldn't pose to be that big of a problem for Thanos. Ra's Al Ghul is known to be immortal due to his complicated and deep backstory that is known to span back 600+ years in the comics.
Thanos was born on the planet Titan with God-like abilities in the species named Eternal. As a product of a high-powered space species, Thanos would defeat less powerful Ra's Al Ghul.
9 CAN'T: Darkseid
Darkseid is one of the most powerful immortal Gods in the DC Universe. He doesn't wither or degrade through time and has strengths that supersede most villains in every single Universe.
Ranging from telepathy to Omega Beams, Darkseid would be nearly impossible for the villain from Titan to defeat. This would be a heavyweight showdown that fans of both Marvel and DC Universe would die to see.
8 CAN DEFEAT: Barbatos
Barbatos of the dark multiverse is an immortal being that turned dark along with the world in Dark Nights: Metal. He has phenomenal capabilities that clash with the greatest heroes of the DC Universe, but in a battle against Thanos, he would fall.
Thanos is a master of absorbing comic energy and harnessing it for his own strength. Barbatos's main ability is expelling cosmic energy upon his victims which would only work in Thanos' favor.
7 CAN'T: Ares
The Greek God Ares would pose to be a major challenge for the popular Marvel supervillain. In a feat against Wonder Woman (both in the comics and the DCEU film), Ares showed clear superiority over the Demigod. Although he has been sent back to the underworld time and time again, Ares is never truly killed and can ultimately never be defeated.
Thanos may be quicker than the Greek God, but he is in no way as powerful.
6 CAN DEFEAT: Doomsday
Doomsday would be a challenge, but overall Thanos could use his cosmic-harnessing abilities to control any force that would be thrown at him in this fight. This is a tough battle, though, because Doomsday has the skills of invulnerability on his side which could possibly block any cosmic attack.
However, in the end, Thanos would be projected to reign as victor solely due to the fact that he has all of the abilities of Doomsday but on a much larger and solar scale.
5 CAN'T: Perpetua
Perpetua is not only known as a Goddess, but she is an all-knowing creator in the Omniverse. She has the ability to merge entire civilizations and can alter the core of creation. Even if Thanos had the Infinity Gauntlet equipped, Perpetua would still win this face-off.
A battle against this DC Goddess would be seemingly impossible for the Marvel villain.
4 CAN DEFEAT: Trigon
Trigon, the demon of the underworld, has several main abilities such as energy projection. Parallel to Barbatos, Thanos would be able to harness this demon-god's abilities and project them back on him.
One thing that would pose to be a problem for Thanos is Trigon's ability of telekinesis manipulation. However, Thanos is headstrong and can resist lapses in memory dysfunction.
3 CAN'T: Anti-Monitor
Anti-Monitor is simply one of the strongest beings in the entire DC Universe. Although some of his main abilities include cosmic conjuration, he has anti-matter harnessing abilities that counteract any flaws to his cosmic conjuration ability.
As Thanos would harness most of his cosmic energy, Anti-Monitor has expulsion and absorption abilities that make it seemingly impossible for a being such as Thanos to stop.
2 CAN DEFEAT: Vandal Savage
One of DC's more popular villainous Gods is Vandal Savage, commonly known for his strike against the aloe-land Atlantis that sent it down below the sea. Vandal Savage is more on Thanos's level in terms of power, and the two match each other in terms of morals.
Although as one of the oldest Gods (50,000 B.C.), Thanos would be able to defeat him in a battle. A strong capability that Vandal Savage acquired over the years is leadership skills which Thanos has proven to slice through time and time again.
1 CAN'T: Grail
Grail is the daughter of the Amazonian assassin Myrina and Darkseid and harnessed all of their abilities. She is the subsequent product matched with Wonder Woman. She not only has the Amazonian skills but also is immortal.
Having acquired all of Darkseid's Omega Beams abilities coupled with her extremely fast makes her an impossible feat for Thanos.