Clefairy, because they just changed the ears colour and it just got worse. It would be much better, if they used the intended blue shiny, made all colours inverted, or just swapped the base and detail colours, but they choose the worst way possible: changed a single minor detail and gave it annoying shade. But the actual worst shiny, without being biased is Pikachu, because, well, it's a mascot and they MUST'VE did something a little bit better, than just making it slightly brighter.
