Thanos and Ultron are two of the greatest and most formidable villains the Avengers have ever faced. When either of the two appears, their presence often creates end-of-the-world level threats for Marvel's most famous supergroup. Both of them inspire fear in even the most powerful heroes and have solidified their places among the top forces to be reckoned with in all of the Marvel Universe. If there was ever a battle between these two titans of villainy, who would emerge the victor?
Ever since Thanos' debut in The Invincible Iron Man #55, he has made a name for himself as a being of immense strength and intelligence. His strength and tendency for ultra-violence have spelled defeat for many characters, both earthbound and cosmic. Thanos has the strength to punch directly through people, as shown in Civil War II #1 when he punches through War Machine's body, as well as his father Mentor in Thanos (2016) #2. Aside from sheer force, Thanos' intellect is one of his greatest weapons. He has been able to outsmart some of the universe's most intelligent beings, most notably in Jim Starlin's The Thanos Quest (1990), where he cleverly bests several cosmic entities in order to gain the six Infinity Gems.
Ultron's greatest power as a villain is his capability to upgrade himself and adapt, making each fight against him a new challenge. Made by Dr. Henry Pym, the original Ant-Man, Ultron possesses the same inventive intuition as his creator. One of Ultron's most powerful features is his adamantium body, making his exterior incredibly durable. Some of Ultron's other powers include super-strength, the ability to fly, and the ability to create powerful energy blasts. Though defeated countless times, Ultron always finds a way to be rebuilt, making him extremely difficult to be completely defeated. Despite these technological advantages though, Ultron is no match for Thanos.
Physically, Ultron's adamantium body and weaponry would make him a worthy opponent in a direct fight against the Mad Titan. Though adamantium is notorious for being among the most durable metals in all the Marvel Universe, it is not guaranteed protection. In Secret Wars (1984) #12, the Hulk puts a dent in Ultron-11's armor with a single punch, allowing space for the Wasp to shrink down and enter Ultron's body to start dismantling him from the inside. Thanos has exhibited numerous times that he can at the very least hold his own in battle against the Hulk, making way for the possibility that Ultron's armor may not be enough to withstand numerous blows from someone with Thanos' strength.
Thanos' superior intellect would also pose a challenge for Hank Pym's manic AI. While one of Ultron's greatest strengths is his technology, it has also proved to be one of his greatest downfalls. Ultron's robotic structure is often exploited by his opponents in battle, causing his defeat. In Brian Michael Bendis' The Mighty Avengers #6, the Avengers defeat a female Ultron by infecting her with a computer virus planted in her body by a shrunk down Ares. In Avengers #68, Hank Pym makes Ultron lose his mind and self-destruct by simply saying the phrase "Thou Shalt Not Kill." Though Ultron's body is extremely durable, it is hard to believe that a being with the level of intelligence that Thanos possesses would be unable to find a way to use Ultron's own programming against him. Thanos has outsmarted some of the wisest beings in all of the universe. Even if Thanos could not directly break Ultron's armor with strength, he certainly possesses the intelligence to find an alternative way of destroying Hank Pym's creation.