I've always thought Magikarp and Gyarados were pretty neat on a conceptual level. You've got a Pokemon that, at an initial glance, seems to be the most useless entry in the entire series. It doesn't start with a damaging move, the damaging moves it can eventually learn are weak and inconsistent, and its stats are awful even compared to other unevolved Pokemon. Many players who didn't know any better cast Magikarp aside as a troll Pokemon. Why would you invest time training something so bad?
Well, as it turns out, those who did invest time training the seemingly useless fish would eventually be rewarded with one of the strongest Pokemon in the entire franchise. This is a really simple yet interesting dynamic that I don't see a whole lot in other games; a monster or character or item that starts out nearly useless, but, if time is invested into mastering it, becomes one of the most potent options in your arsenal.
Gyarados is also a Pokemon that just can't stop winning when it comes to its competitive career. Gen 2 introduced Hidden Power, which gave it a decentish Flying STAB to leverage its higher Attack stat with. Gen 3 gave it Intimidate and Dragon Dance, turning it into a deadly sweeper. Gen 4 introduced the physical/special split, finally giving it physical Water moves to work with. Gen 5 gave it Moxie, another fantastic ability option that makes it easier to snowball into sweeps. Gen 6 gave it a Mega Evolution that lets it bypass many of its usual checks. Gen 7 gave it access to Flyinium-Z, letting it turn Bounce into a one-time super-powered Flying move. Gen 8 introduced Dynamax, which Gyarados happens to be one of the best abusers of thanks to STAB Max Airstream and Moxie letting it get pseudo-Dragon Dance boosts just by KOing opponents.
I wonder what new toys Gen 9 will bring our classic sea serpent murder machine.