"What makes a 'good' shiny?"
In my opinion, it's a sprite that's different enough than the original, yet still reflects on the typing of the pokemon. An example would be Chandelure - it's normally purple/blue, yet the shiny sprite is orange/red.
I like this because it's dealing with fire (orange/red), yet looks vastly different than the original sprite, which is the blue color.
Don't even get me started on how much I HATE Kangaskhan's shiny sprite.
Ugh.
For me, I don't really care if it doesn't match the typing, any shiny sprite is good as long as it is clearly different than the original color.
However, there are also a few shinies whose colors are a bit disappointing, because while it is clearly different, but it's still not enough, because it's just different shades, not exactly different color.
A great example of this would be Shiny Blaziken. The colors are clearly darker than the original, but it's also very obvious that they are just darker shades, they don't change color.
It's sad, because Shiny Torchic has such an amazing shiny sprite, with the yellow and orange being swapped.
Then there are also a few shinies whose colors are great, because it is clearly different, but unfortunately their previous evolution has a better shiny color.
A great example of this would be Shiny Rapidash, Shiny Drapion, and Shiny Zoroark. To be honest I much prefer Shiny Ponyta's blue flame, Shiny Skorupi's bright red body, and Shiny Zorua's bright neon blue. Especially Shiny Zorua VS Shiny Zoroark, that bright neon blue on black works so much better than purple on black.
And yeah, Shiny Kangaskhan's color sucks. But Shiny Garchomp will always be the worst shiny color ever.