When I was a kid, I found Colossus to be the most boring of the X-Men. He was a big guy who could turn into steel, giving the team some muscle (every team needs a Hulk/Thor/Thing dude to knock down buildings and stuff), but he was dull. A quiet, sheltered Russian kid who did a lot of standing around and being shocked when Storm walked around naked, a thick hick who made Wolverine seem cosmopolitan and even Cyclops vaguely dynamic. I think his creepy relationship with my comic-crush Kitty Pryde made me resent him, especially when he flaked out and broke her heart in the aftermath of Secret Wars (more on that later).
But Colossus was the glue of perhaps the greatest superhero team of all time. The Wolverine-Nightcrawler-Storm-Colossus (sometimes Cyclops and sometimes Kitty) X-Men in the hands of Claremont and Byrne were my introduction to comics, and still the gold standard to which I compare all other Marvel efforts, which almost universally fall short. You're talking about the Dark Phoenix Saga, Days of Future Past, the Brood saga, one of the best Magneto arcs ever told, all crammed into fewer than 100 issues (some of which included artists other than Byrne, but all of which represents Claremont's absolute pinnacle). Wolverine was the breakout star, Nightcrawler the tragic clown, Storm the den mother, and Colossus the steadfast pillar of strength, both literally and figuratively. We all have buddies like Piotr Rasputin (and if we don't, we should): reliable, loyal, powerful, yet shunning the spotlight, content to support his flashier teammates. Colossus is always the guy throwing the Fastball Special, rarely the guy soaring through the air.
And yet a couple of my favorite comic sequences of all time feature Colossus. The first took place following the heroes' return from Battleworld, when Piotr confronts Kitty about his intergalactic infidelity with the Targaryen-looking healer Zsaji. He breaks up with her, which sounds ephemeral but was a seminal moment in the book's history. That afternoon Wolverine (with Nightcrawler along as chaperone) takes his beefy but misguided buddy to a downtown bar to drink, yell at him, and maybe smack him around a little. Lucky for Logan, the Juggernaut just so happens to be in the same watering hole, and the two big guys go after it. It's a great character moment for everyone involved, and the upshot is that for once, "Petey Pureheart" is the guy playing the cad, making the bad choice, betraying a teammate. This is perhaps the single moment in time separating "Before" from "After" in X-Men history. The greatness of the All-New mutant team is on borrowed time.
Bookending this moment is one at the true end of the line, during the Mutant Massacre that ended with Kitty and Nightcrawler and even Colossus so messed up that the team would irreparably shatter. Confronted by the death and carnage of the Morlock tunnels, and by the callous, murderous Marauders, Colossus snaps. The peaceful artist boy from Ust-Odinski, who revered life and would not even contemplate killing the Brood parasite within him, takes the villain Riptide by the throat and snaps his neck. Colossus is all grown up, but he's a changed man, a sadder, darker creature. Comics were moving that way, chasing the moral ambiguity of the popular anti-heroes of the time, and there was no room for a man of steel with a heart of gold.