Colossus is stronger/more durable, but Ares has Adamantine weapons, which coupled with Ares' strength might allow them to cut him. Which could obviously end badly.
Well with a weapon Ares has to be the favorite. It's not really fair unless Colossus has one, but Colossus has fought with bo-staffs, swords, and axes before. He was trained by Nightcrawler with a sword.
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The Axe broke on Ultron's shield, so it can't be adamantium. However, Ares shud be the better fighter of the 2 while Colossus is the stronger and more durable one. The strength difference shouldn't be all that big tho. Tho I can see that the durability being the big difference here. However, if they have weapons that can cut each other, the durability becomes much less of an issue.
H2H (no weapons) - Colussus wins
With weapons - Ares wins
Unless Ares has his Adamatine axe, he isn't cutting Colossus.
Ares should be an extremely talented warrior, but he runs in screaming at the top of his lungs and charging at his opponent. His like a Wolverine/Rhino fusion in that sense. All that supposed skill doesn't matter, if he doesn't decide to use it, and just brawls. Also Colossus is also experienced and trained in fighting, but I'm assuming no where near Ares' experience and so on.