Since originally Pokémon Red & Blue released on the Nintendo Game Boy, none of the sprites featured color. This means that is wasn't until Pokémon Silver & Gold that players knew what a shiny pokémon was. The variant colored versions of pokémon ending up being super rare, and most trainers have either never encountered one or didn't pay attention and treated it like any other wild battle. Let's look at the shiny forms of the pokémon from the Kanto region featured in Pokémon Red & Blue and rank the 10 best ones.
Muk and Grimer are pokémon that are based on sludge and waste which means that their baseline purple coloring is pretty fitting. When you think about what color their shiny versions can be, green is pretty spot on of what other colors you'd associate with waste. Brown is another color opportunity they had put they probably wanted to avoid the comparison to human waste. Funny enough, the Alolan versions of Grimer and Muk have their green coloring turn to purple in its shiny form.
Most young boys love dinosaurs and several young girls love colors like purple or pink. The shiny version of Aerodactyl brings those two worlds together and has the ancient fossil pokémon embracing a colorful skin. The normal version of Aerofactyl is cool because it resembles a dinosaur, but there's not a lot of personality to its design outside of that. With its shiny version, it's able to stand out in a way that makes it attractive to way more trainers than it normally is.
Slowbro is often joked about as being one of the largest dunces among all pokémon. Though this isn't true in the games but is in all other forms of media, Slowpoke is unique in that it evolves into Slowbro only if a Shellder bites its tail. This means that the only difference between a Slowpoke and a Slowbro is that one is in agonizing pain. If you know you're going to have a suffering pokémon by your side, then you should make sure it looks cool. Be the talk of the town with a shiny purple Slowbro.
Very rarely do shiny versions of pokémon turn out better than their base forms. Sure, some are just as cool or different in a way that gives it new life, but rarely are they assuredly better. Starmie may be one of the few cases in which there's an argument to be made that its shiny version is cooler than its normal coloring. Nostalgia for the anime will still pull you towards Misty's purple Starmie, but the blue and red come together on the shiny form in a really solid and captivating way.
Soon, we'll be talking about Mew's shiny form which turns the legendary cat-like pokémon from pink to blue, but its now time to talk about Vaporeon who makes that color change in the opposite direction. Instead of being blue to reinforce that it is the water evolution of Eevee, Vaporeon dawns a hue of pink in its shiny form displaying all sorts of feminine energy. The colors often associated with mermaids are green, blue, and pink so it makes sense that Vaporeon would only appear in those colors.
So, not only is Mew one of the rarest pokémon to own in the games due to it only being available at mall events in the late 90s and early 2000s, but it has a shiny form. Shiny versions of legendary pokémon have always seemed odd considered these legendaries are supposed to be one-of-a-kind. Whereas you can understand that a high population of a single species of pokémon can have weird versions, having a shiny version of a single entity would just record as its normal version since there's only one of them. Mew's shiny version is blue, by the way.
Dragonite is arguably the most overlooked and misunderstood pokémon in all of Kanto. A lot of that has to do with the fact that he looks like a goofier version of Charizard in many ways. He's designed more like a recurring character on the kid's cartoon Dragon Tales.
All that being said, he's powerful and capable of learning a good set of moves. His shiny form turns his peach color to a wasabi-like green, which helps him stand apart from everyone's favorite fire-starter pokémon from Kanto.
Once a kid learns about blue flames and how they burn hotter than normal ones, they often become obsessed with it. Rapidash is a pokémon that has literal flames emanating off of its body, so the idea of having one that's distinctly blue and not red is rather enticing. If you assume that the history of the Pokémon is parallel with ours but features pokémon then you can imagine classic cavalry battles in which the general or maybe royalty rode into battle on the back of a shiny Rapidash.
Charizard has always been one of the coolest pokémon and has stood the test of time as a franchise icon. The orange dragon design works simply because people love dragons.
When the shiny version was discovered and people quickly realized they could own a black Charizard, it blew people's minds. Those who struggled to come across or acquire a shiny Charmander were at least given a chance to make their own Charizard's black for a moment with the Mega Charizard X form.
Not accounting for luck, the shiny red version of Gyarados was 98% of pokémon trainers' first exposure to shiny pokémon. It can be found in Pokémon Silver & Gold in the Lake of Rage, and players only get one chance to catch it. Many trainers had no previous knowledge of the encounter, so it was sort of sprung on them and they had to strategize accordingly if they planned to catch it.
Gyarados is one of the coolest pokémon to begin with, so giving him a solid recoloring of his shiny version was very smart by the people at GameFreak.