Here's some of my fave shinies ever:
Corsola - I love the blue on it. If I were to ever shiny hunt for a Pokemon I never caught a shiny of yet, it'd definitely be a Corsola who I can just stare at its beauty.
Chandelure - Not only one of my fave 5th Gen Pokemon ever, but also one of my fave shinies from there. Orange flames look really beautiful.
Miltank - It's one of my fave 2nd Gen Pokemon, and I now have a shiny one of my own in X.
Aipom - Who can't resist a pink monkey? It's one of my fave pink shinies.
Vivillon - Particularly the Icy Snow ones, they look nice when shiny. Sadly my region's form, which is the River Pattern, doesn't look as great compared to the shiny Icy Snow Vivillon.
Whimsicott - I love the white + blue combination. It looks even cuter that way!
Pumpkaboo - Purple and black are two colors that look so nice on the little pumpkin.
Sylveon - The normal color is nice, but I think its shiny is even cuter.
Dedenne - Dat brown and yellow. It's a nice shade too.
Hawlucha - One of the wildest color combinations ever seen on a shiny. Makes me really want one someday!
As for my least faves, like what was mentioned earlier, any shiny whose color looks too close to the normal color. Garchomp and Blissey being notable ones here.