Oh boy. What's cool about Colossus?
He manifested his powers saving his sister. Boom, he's got the hero magic. This farm boy from Russia comes on over to America, learns another language, and immediately helps rescue the A-team of the X-men at the time, because they got themselves captured. Then he proceeds to just be this giant teddy bear of a dude who doesn't really have beef with anybody and just wants to get along.. kind of the personification of Xavier's ideals, even though he gets zero credit for it because there's more popular X-men like Jean Grey, Scott, and Logan to be angsty about their personal lives (sry to peeps that like them tho I do too, just saiyan'). When this dude isn't lifting the blackbird for fun because Kitty told him to he's painting to try and be chill because he doesn't want to become a big green rage monster. He's fought gods (Thor), he's fought unstoppable villains (Juggernaut), he's fought battles he
knows he will lose (The Hulk) just because It's The Right Thing To Do and because at that moment, if he didn't do it, no one else would be able to. He does it all without a bunch of personal angst about how hard his life is or how many battles scars he's got or how many times he's been knocked into canada from New York because some jerk decided to play fisticuffs at a school for mutant children (Again, the hulk).
He willingly lost even when possessed by a destruction-loving demon as the avatar of Cyttorak for the sake of not endangering Utopia (the island) and New Atlantis. He used the cosmic powers of only
one-fifth of the Phoenix Force to denuclearize the planet, solve world hunger, and negotiate with the Electric Legion to become a power source for Eastern Europe. Take that, Jean Grey. When was the last time she tried to talk down a gang of pure energy beings into helping instead of killing people, much less did some real good with her cosmic powers that wasn't trying not to lose control? (to be fair, he only had a portion.. but also to be fair, he only had a portion and was exponentially more helpful with it.)
When he isn't being written as some randomly-raging jealousy monster for someone's self-insert (big shoutout to Ellis for that), he's just a compassionate, introverted, tower of muscle and kindness that just wants to find some semblance of peace for himself and those he cares for. Of course, that means that he gets manipulated by his own sister into becoming the juggernaut, having her go behind his back to interfere with his wedding/etc, but hey, the fact that he keeps forgiving Illyana despite her many mistakes is just kind'a testament to his character. Some people see that as a pushover.. but it's easy to be upset and kick someone out of your life when they do you wrong. It's much harder to forgive and continue caring for them (especially when it isn't just because of a lack of options, but because they're the only family each other has and that matters to him).
Best of all, he does this kind of under the radar of most people. He is never the most popular, or even second most usually, he's just kind of
there because he rarely acts out or is ever dramatic. His powers aren't flashy (though they can be reflective! and impressive, when he stops cars and ignores bullets and bombs and such). he's just a tankbro being that dependable strongman who's always willing to help whenever his friends need him. He doesn't go out of his way to interfere in people's lives, but if someone calls, he's got their back.
Also, he's got actually some of the more iconic aspects of comicbook-ness there are. His signature move is known by
everyone, even those that don't know it's
his, and it's been replicated and copied a hundred times over just because it's the epitome of cool:
The Fastball Special.
It has its own wiki page, it's that good.
And then there's this iconic cover drawn by John Cassaday which is one of the best looking covers of all time, imo (though clearly I'm biased), and certainly one of the best ones of the more recent comics:
Which is to say nothing of the recent Deadpool movies in which he's portrayed awesomely also, or to go in too deeply to the fact that he's still one of the strongest and toughest mutants on the planet but very rarely ever flexes his potential, and usually does it on the defensive, for the sake of someone else's well-being, when doing things like holding up buildings and the like.