The Juggernaut has been one of the X-Men's oldest and most powerful foes since Stan Lee and Jack Kirby created him in X-Men #4. When he was born, Professor Xavier's step-brother was merely a normal human named Cain Marko, but he was granted superhuman strength and endurance by the demonic entity Cyttorak when he touched a magical gem.
Since then, the Juggernaut has been an almost unstoppable villain in his countless battles with the X-Men and other powerful Marvel heroes including the Mighty Thor and the Incredible Hulk. Although Cain Marko is the original and most well-known of those who have held the power of the Juggernaut, he is by no means the most powerful. In reality, that designation goes to one of his oldest sparring partners, the X-Men's Colossus.
Since Piotr Rasputin joined the X-Men in Len Wein and Dave Cockrum's Giant-Size X-Men #1, his super-strength and nearly impenetrable steel skin have made him one of the team's heaviest hitters. When his team has had to face the Juggernaut, Colossus is usually one of the first mutants to battle the villain head-on, since he's one of the only X-Men who can give and take blows on the Juggernaut's level.
In 2011's Fear Itself event crossover, Cain Marko is transformed into Kuurth: Breaker of Stone when he picks up a hammer enchanted by Cul, the Asgardian Serpent God of Fear. Kuurth is an upgraded version of Juggernaut with additional strength and durability, which the X-Men discover as they unsuccessfully send wave after wave against him in hopes of stopping his violent rampage through San Francisco.
Eventually, Cyclops enacts a plan that involves Magik teleporting Kitty Pryde and Colossus into Cyttorak's dark realm to inform him of what is going on. Cyttorak is offended by Juggernaut's acceptance of the Asgardian Serpent's power and elects to strip his powers from the Juggernaut, intending to bestow the mystical power to Magik.
However, things don't go exactly as planned. At the moment the power is being transferred, Colossus steps forward in an effort to protect his sister. He intercepts the power of Cyttorak and is transformed into the Unstoppable Colossus.
With the power of Cyttorak added to his mutant powers, Colossus became one the strongest hosts of the Juggernaut to date. With relative ease, the Unstoppable Colossus proves to be the only force capable of halting Marko and pushing him back away from the city of San Francisco in the process.
While his newly acquired power made him more formidable than ever, it also provided complications for Colossus. His longtime love interest, Kitty Pryde, broke up with him because of his constant need to sacrifice himself, and Colossus began to find it difficult to resist the destructive urges brought on by his new power. After his fight with Marko, Colossus stayed with Cyclops' team on Utopia rather than returning to the X-Men's school in Westchester County for fear of losing control of his emotions and possibly harming the children who resided at the school. Still fearing the worse, Colossus later requested to be placed in an isolated cell, unleashed only when his powers were needed.
Still in possession of the Juggernaut's power, Colossus was a key figure in the X-Men's battle with the Avengers in the Avengers Vs. X-Men crossover, where he also absorbed a portion of the almighty Phoenix Force. Unlike Cain Marko's situation in Fear Itself, Cyttorak was perfectly happy to let Colossus simultaneously wield the Phoenix Force and keep his Juggernaut powers since he was so violent in that era. Throughout the crossover, Colossus took on several members of the Avengers like the Red Hulk and the Thing.
After Spider-Man tricked Colossus and Magik into attacking each other, the two siblings used up their respective Phoenix powers. And not long after that, Magik used her abilities to remove Colossus' Juggernaut powers as well.
Since then, Cain Marko has reclaimed his Juggernaut's power and Cyttorak's favor, having lost Fear Itself upgrades long ago. While Colossus ultimately only possessed the powers of the Juggernaut for a relatively brief period of time, the combination of Cyttorak's mystical powers, the Phoenix's cosmic strength and his own baseline mutant abilities distinguished him as the most powerful incarnation of Cyttorak's avatar, and one of the most potentially powerful beings to ever walk the Earth in the Marvel Universe.