Does anyone now how they handle their shinies? It feels like the shiny pokemon's coloration is done by a program with the cool looking one's just being happy accidents.
Turns out, that's actually kind of true.
According to this, back when shinies were first introduced in GSC, there technical and memories issues that prevented shinies from having a more diverse color palette. Pokemon were only colored with one, or two colors on top of an already black and white sprite. A shiny pokemon wasn't designed to look cool, or appealing, but rather its coloring was decided upon with what color, or colors could be easily swapped into the 'mon's already existing code.
The technical and memory issues stopped being a problem in Gen 3, but Gamefreak being Gamefreak decided to keep the stupid lazy way of designing shinies. This continued all the way up until Gen fucking 6, when The Pokemon Company (not Gamefreak) actually had a say in how shinies would look. This is why the shinies for some Megas like Mega Gengar look so much better than their shiny base forms. The Pokemon Company decided that, not Gamefreak.
Gamefreak themselves would not actually start designing shinies until Gen 7 when they finally decided to start designing shinies by hand, and not just by swapping in whatever color worked with their broken-ass spaghetti code. This is why modern shinies look so much more appealing than most old ones.
As for why some old shinies look particularly good, most of the 'mons in question just got lucky, but it is speculated that at least a few like Gyarados, and Charizard were hand-designed to look cool, and not completely ugly and uninspired like most shinies do, unfortunately.