Iron Man's most powerful armor was seen in Avengers Endgame, but he wasn't the one wearing it.
In all, Iron Man created 85 armors in the MCU, which is much more than in the comic books Marvel. Tony Stark had said he wanted to develop something to defend Earth in case Thanos returned and he did. He built his own Iron Legion. If at first glance it is difficult to distinguish his armor, they are no less different from each other. There is one in particular which is said to be much more powerful than the others, and which appeared in Avengers Endgame
This is indeed what the site emphasizes Screen Rant , which also recalls the exploits of Pepper in the Rescue armor, identifiable by its blue color. While wearing her gift, Pepper is able to stand up to Thanos, defeat several ships in his fleet , and his firepower even seems superior to that of Iron Man. Tony Stark created the Iron Legion in order to help the Avengers defeat their enemies, but also to reduce losses as much as possible on the civilian side. His idea came to him after the Battle of New York, fueled by his fear that his world and his would be destroyed in a surprise attack. So it's no wonder he entrusted his most powerful armor to his wife. He probably wanted to make sure that whatever happened, with or without him, she could stay safe.