My best shiny I think is tied. The first is my Shiny Gallade and the other is my Shiny Meloetta egg. I might get a better shiny in the future though so this might change
perhaps the origin giratina i got recently. or my lvl 100 flygon. OH or the cresselia too can't believe i forgot that one (i guess though technically the rarest one is the anniversary azelf)
Update/edit/correction: I am so foolish to have suggested my nine Shiny Fletchlings were my best shinies. I forgot to account for future shiny Fletchlings. I have 10 now.
I think the shiny Mimikyu I got myself is pretty cool! I actually have one in Sw/Sh, so that's pretty nifty as well.