He was the one who started it all. Will he be the one to end it? While many fans are speculating that a heavyweight hero like Hulk or Captain Marvel will be the one to ultimately bring down the Mad Titan, there are numerous reasons why Thanos should fear Iron Man the most.
Stark first caught Thanos' attention after thwarting his original invasion of Earth back in the first Avengers film. Even though Thanos himself wasn't present at the invasion, Stark's heroic actions still ultimately foiled the Mad Titan's plans of conquering Earth and claiming the Tesseract for himself. A being as driven and powerful as Thanos would surely make a point to remember anyone tenacious enough to defy his whims. This is exactly why Thanos knew who Tony Stark was during Avengers: Infinity War, as confirmed by Joe Russo during the commentary track for the film's home release. Russo went on to describe the "existential connection" between the two that has stemmed from that first conflict. The fact that Stark is the only Avenger that Thanos knows and addresses by name already shows that the Mad Titan views him as a potential threat.
Back in Age of Ultron, Tony Stark actually predicted the events of Infinity War with frightening accuracy. During a heated exchange with Steve Rogers, Tony's concerns about an unstoppable threat from space not only referenced the Battle of New York, but also foreshadowed the arrival of the Mad Titan. Not only did Stark predict the title of Endgame during this verbal tirade, he also foretold their defeat at the hands of Thanos. The Avengers eventually give it all they have, but they weren't enough to stop the Mad Titan from collecting the Infinity Stones. Rogers might be the courageous leader who the heroes look up to in the darkest of times, but Stark is the only realist on the team trying to plan ahead before disaster strikes. Tony's uncanny ability to realistically anticipate what the future holds is another reason Thanos should be wary of him. Long ago, Thanos foresaw the downfall of his own civilization due to overpopulation and exhaustion of resources. Despite his best efforts, the people of Titan wouldn't listen to his call for action, understandably, due to his radical idea of eliminating half the population. Keep watching the video to see why Thanos should fear Iron Man most! #IronMan#Endgame#MCU#IronManEndgame He's stopped Thanos before | 0:14 Predicting the threat | 1:02 They're both futurists | 2:00 Stark Adapts | 2:45 Upgrade to the stone age | 3:59 Nebula's knowledge | 4:39 It all started with Iron Man | 5:21 Thanos tried to kill him | 6:25 Two sides of the same coin | 7:06 Entertainment