The primary truth of the question Who would win in a fight? is that any character could honestly beat any other character if the writing were strong enough. Buuuuuuuuut that doesn't make it less fun to speculate.
Hey, Iron Man is pretty great, but which DC characters could pummel his ass into the ground? Here are our brackets.
First up, our challenger, Anthony "Tony" Stark, the Iron Man. Stark is smart, physically fit, and has a vast amount of money and resources. And then there's the whole fleet of giant murderous robot suits thing. But that doesn't make you invincible, and if Stark ever steps into the DC Universe, here's who would shove him back through that portal.
Look. Tony Stark is a smart man. His suits are powerful pieces of technology. But Superman is friggin' Superman, and he will demolish you. He will pop you out of that tin can you call armor like he's squeezing toothpaste out of a tube.
Iron Man might work hard in preparation, but the Man of Steel will nevertheless mess him up.
So what will he do? Power a suit with Kryptonite and beat Superman that way? Man, it's a good thing Superman doesn't have a history of fighting a robot with a Kryptonite heart or something. A robot made of…I don't know…metal? Oh, man, it's a good thing that guy can't possibly exist and show why Superman would win this fight.
All kidding aside, Superman is used to fighting people who leverage Kryptonite against him, and with the low-level cancer-causing effects of Green K, odds are that Tony wouldn't want to use it for long, which would rob it of most of its effects.