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Diancie's a little boring but it does have appeal, the soft and polished colour of the crystals against a rough rocky grey works strangely well. Hoopa's interesting in what it does and its ability to radically shift in tone between a rather zany cartoon imp to a many-armed beast of destruction is, like Diancie, a surprisingly natural blend of two opposing things - the two feel coherent together. Volcanion's a precious komainu and again presents this theme of opposing things blending together but in a much more mundane form with its steam powers combining Water and Fire.
Magearna's much more interesting than Diancie with its steampunk aesthetic and complements the previous Volcanion nicely, with a lot to explore within its design that really brings it to life which is an impressive feat against its robotic nature and static face. Marshadow's got a very simple, basic charm but the only point of interest I have towards it personally is a small similarity to Mimikyu (both are black, shadowy creatures that stay hidden and mimic others). There's nothing good to say about Zeraora.
Overall, I much prefer Gen 6's mythicals but I'd substitute Diancie for Magearna if I could.