While Iron Man has built plenty of deadly things in the past, his adamantium Stark Sentinels take the crown as his most dangerous invention. This is because Tony Stark was manipulated into creating them using all the data he gathered on Earth's heroes during and after Civil War. While this means they'd technically make sense in the MCU, it's unlikely movie fans will ever see them, as they were simply too powerful to work in that universe. After all, the perfected versions of the mutant-hunting robots were perfectly designed to counter every hero power in existence and imprison the Avengers, X-Men, and others so they could be executed by the Red Skull.
Following Charles Xavier's death, the Red Skull was able to steal his telepathic brain and use it to promote hatred and violence across the world. This came to a head in Avengers & X-Men: AXIS #1, when it emerged that the villain had used his powers to push Tony into constructing two vast Sentinel robots from impenetrable adamantium - Sentinels which Red Skull then used to begin disposing of his enemies, leaving the planet without its vital protectors.
The story - by Rick Remender and Adam Kubert - reveals that the Sentinels are equipped with Pym Particles, allowing them to shrink Marvel's heroes and gradually take them out of the fight, protected from their superpowers by unbreakable bodies. The Sentinels are also in possession of all the data Tony gathered on his fellow heroes, giving them unparalleled insight into their true identities, powers, and tactics. Thankfully, Magneto is able to defeat the Sentinels by summoning an army of villains - who the robots haven't been programmed to combat - but they would unquestionably have bested the Avengers and X-Men without this last-second gambit. Sadly, the spell that follows - which flips the moral alignments of those present - means that one of the Sentinels is acquired by another major threat.
This Sentinel returns in Avengers: Rage of Ultron, where it's rescued from A.I.M. by the radical AI-rights group the Descendants. While Hank Pym is able to finally end the threat of the adamantium Sentinel, he is forced to kill it and the Descendants to do so, using a device he has designed to finally end the threat of Ultron. Hank admits that the Stark Sentinel would have made the Descendants unstoppable had it been able to share its Civil War files, and it's again clear that the heroes stand no chance in direct combat against it, even without its use of Pym Particles to pick them off one by one.
When he asked other heroes to register with the government, Tony Stark swore that he would keep their secrets safe. He tried his best to live up to that promise, even erasing his own memories to stop Norman Osborn getting the information, but ultimately he failed to account for the myriad possibilities of the Marvel Universe. In a world of magic, alien technology, and mutant telepathy, there's no surefire way to keep a secret, and Red Skull was able to use Iron Man's knowledge to construct a threat the Avengers and X-Men couldn't handle short of executing sentient life.
Tony Stark's arrogance is his fatal floor, as the "ultimate futurist" fails to appreciate that the future won't play out as he predicts. In the case of his adamantium Sentinels, Iron Man was tricked into creating the ultimate weapon against his friends and allies, but it's one that would never have been possible if not for his misguided surety that he could never be compromised.