Iron Man would win, but just barely.
The “Implants” section of the link above shows biological enhancements to even the most basic marines that would make them very tough, even without their suits. With their suits one, they would be even harder to kill. Their suits are supposedly able to withstand in a variety of harsh environments, like Iron man, so they would both be able to survive in freezing, boiling, or low-oxygen environments.
That means that with basic Ultramarine Armor and the biological enhancements, they would be relatively difficult to kill.
Unfortunately, most of the stuff I’ve seen doesn’t actually have Ultramarines doing anything or fighting, and the wikias don’t list their weapons, so I have no idea what their standard weapons are, or how many they can carry at once.
In that case, I assigned basic but powerful weapons to an Ultramarine:
A Chainsword http://spacemarine.wikia.com/wiki/Heavy_Bolter_(Space_Marine)
A Bolt Pistol http://spacemarine.wikia.com/wiki/Bolt_Pistol_(Space_Marine)
And A Heavy Bolter http://spacemarine.wikia.com/wiki/Heavy_Bolter_(Space_Marine)
The downsides to the weapons are that some weapons reportedly jam for no reason, and require constant maintenance and “ancient litanies to appease their spirits”. Iron Man doesn’t need to pray for his guns to work.
Now we take a look at Iron Man. Unlike Ultramarines, he has a little more substantial information, with identifiable evidence of his weapons and abilities. In his most basic, fully functional suit suit fro the first movie, he has super-sonic flight, tank-busting missiles, auto-targeting systems guided by a super-intelligent on-board AI, adaptabiliy to low/zero--oxygen environment, and most importantly, the arc-reactor that fuels his repulsor tech. His hand-mounted repulsors can, as seen in the movies, send people flying through walls. His chest-mounted repulsor can supposedly blast through solid metal, which could possibly (but not likely) kill an ultramarine at point-blank range. In addition, he is repeatedly seen being shot at with high-powered weapons, even sustaining a direct hit mid-flight from a tank’s fire, but is still able to survive and reduce the tank to rubble with one small arm-mounted missile the size of a banana.
In the end, all their technological and biological abilities would, for the most part, balance out. Iron Man’s only true advantage is his efficiency and mobility. He has a near-unlimited power source, his super-intelligent AI would help with avoiding certain attacks, his super-sonic flight speed and maneuverability would also help, and his significantly smaller size would make him a much harder target to hit than Ultramarines, who, with their massive suits and biological enhancements, would be a minimum of 8 feet tall. And even then, it would be a pretty close call.
I don’t want to type this much ever again.