For me it's Emboar. It has such an awesome colour palette and both the orange and blue work together so well. Gotta be one of the most appealing shinies for sure. Honourable mentions: Spoiler So, what's your favourite shiny starter?
For some reason, I really love the purplish pink of the shiny Mudkip line. I also really like the Fennekin line (the purple goes well with the witch theme) the Rowlet line (I love the dark teal color), and the Tododile line (The simple change from red spikes to blue looks really neat.)