In many ways, Kang seems to be responsible for much of what is going on in the main MCU timeline, to the point where he seems to be responsible for the route some characters have taken through the story to date. “I paved the road... you just walked down it”, he said, after being confronted by Loki and Sylvie in Loki season 1, episode 6 “For All Time. Always”. This would mean Kang – or at least the first iteration that has appeared on screen so far, “He Who Remains” - would have most likely been aware of Iron Man’s journey, his actions in the MCU and his sacrifice at the end of Avengers: Endgame.
Tony Stark seemed to have been the only Avenger aware of the repercussions of messing with time travel. In Avengers: Endgame he said, “you mess with time, it tends to mess back”. While it’s not clear what he is referring to, it at least indicates Stark had some experience with the dangers of time travel. Not only is Iron Man a formidable opponent, having drawn blood from Thanos and ultimately defeating him with the largest sacrifice made in the MCU to date. He also seems to be the only Avenger with the foresight, knowledge and experience with time travel to go up against Kang. It’s for this reason Kang might have been fearful of Iron Man, potentially even laying the road that removed him from the equation.
Time is Kang’s domain. He is a time-traveling, multi-variant supervillain and the one iteration audiences have met so far managed to control the flow of time and space, taming the universe into one single strand. The evil version likely to pop up in the future MCU will be the genius Nathaniel Richards, Kang the Conqueror, once again portrayed by Majors. In the comics Kang would use his "time-ship" to travel through time, gathering technologies and using them to prove himself as more than just an academic, battling the Avengers and other characters multiple times along the way. The one person he won’t want to contend with, is someone who has any grasp of time. And Tony Stark not only referenced the dangers of messing with it, but was essential in building the time machine that was eventually used to defeat Thanos.
In the comics, Richards has no superpowers per se, he is simply a genius who uses his knowledge of time travel to develop himself into one of Marvel’s greatest supervillains. His most dangerous adversary would need to be someone equally intelligent, and Stark fits the bill. Utilizing his foresight into the dangers of time travel, and once the threat of Kang has been fully realized, Stark would’ve no doubt come up with a plan to defeat Kang. It would be unsurprising then that Iron Man was the only Avenger Kang feared. Perhaps then it’s unfortunate that Iron Man no longer exists (at least, not in the main MCU timeline) thanks to the events of Avengers: Endgame.