Thanos could also win in a fanfiction. He has incredible strength and the power of the Infinity Stones at his disposal. In a lot of fanfictions, the power levels are often exaggerated. Thanos could simply use the Reality Stone to make Iron Man's weapons ineffective or use the Time Stone to freeze Iron Man in time and then defeat him. His brute strength alone is enough to overpower Iron Man in a direct confrontation. And his experience in battles across the universe gives him an advantage over Iron Man who mainly operates on Earth.
One interesting plot twist could be that Iron Man discovers a hidden power within his suit that can counter the Infinity Stones. Maybe it was something he unknowingly built into it long ago.
Iron Man uses his ingenuity and advanced technology to outsmart Thanos. He comes up with a clever plan and deploys some powerful weapons and strategies.
In the comics, Thanos defeats Iron Man through a combination of his strength, energy blasts, and strategic planning. It's a fierce and intense confrontation.
It depends on the specific fanfiction. In some stories, Superior Iron Man might be more powerful because he's willing to push the limits of his technology and his morals further. But in others, Iron Man could be more powerful due to his experience and his ability to work well with others.