My favorite Pokémon and shiny is Porygon-Z. I just love its color scheme for both. How about your viewer?
I've always liked Shiny Swellow ever since I saw it in the anime XD I think it was Winona who had one? The silvery blue was cool.
My favourite has to be Braviary. I really love the use of blue and brown. It really looks nice. Of course I may be a bit biased with this one because Braviary is my favourite Flying-Type mons (along with Mandibuzz), and a shiny one was a member on my Ultra Moon team Zangoose, Lopunny, Furfrou, Toucannon, Furret, Audino, and Bewear also have nice shinies on my opinion. Spoiler: Shinies EDIT: added more I like.
I think just about every shiny bird Pokémon looks great. Aside from those, Buneary and Lopunny's pink fur looks like cotton candy.
I really like shiny eevee. I know it's just silver, but I had one for so long and I got attached to the look.
I'm not gonna lie, I don't like a lot of normal type shinies. Either because for a lot of them, there's barely a difference in shade, or because they have really ugly colors/color combos are used. With that in mind, my favorites are... Meloetta Toucannon Lopunny Diggersby Zangoose They're all pretty cool shinies, in my book. Especially Meloetta.
Mahoudino (Audino). The magical girl outfit looks great in a shade of purple just as it does in pink. I can't tell you a pure Normal type favorite for shinies otherwise, though...
Zangoose and Porygon-Z are essentially my favourite ones because their shiny colours are blue which is a good shiny colour. :>
Shiny Ambipom. It's a nice shade of pink and I feel like it works. And any normal type Pokémon that is a color other than gray or brown is a winner in my book.
I would have to say Zangoose, it just looks so epic with that lightning blue streak across its chest.