Colossus is the most under-rated powerhouse in the Marvel or any other universe.
He is a victim of negligent writing not lack of power. Back in Uncanny Annual #7 Colossus took down She-Hulk with no problem. She threw a punch and he simply caught her fist and tossed her aside. When he was still a teen-ager he took on Gladiator and stood his ground (though he did not win the battle they toppled a whole temple down upon themselves) he put up a better fight against him than the entire F.F. He's the only X-Man that can stand up to a fully powered Juggernaut by himself (and now that the Juggernaut isn't completely powered by the gem he not so unstoppable) even Hulk has never really beaten a fully powered Juggernaut in a slugfest. More than once he has been described as one of the most powerful beings on earth. Peter has always held back due to his easy going nature, but after all he's been through he's not so easy going as he once was. Prof. X described him in Uncanny #279 as being as powerful as he is unstoppable. As for the Ultimate counterparts, they are just pale immitators. There is no way Ultimate sissy is taking down the "Real Deal"! 
Ultimate was held off by the Falcon with a gun aimed at his eye. The real Colossus has no such limitations, he is invulnerable down to his eyeballs! I also seem to remember Ultimate being taken down by group of physically inferior beings at a party or something, which just goes to show that there are inconsistencies in his displays of power too.
That being said Ultimate Colossus is my favorite Ultimate hero.
