I think Ho-Oh is the better shiny due to it being more thematically approriate, it being a Phoenix and a Fire-type. It's shiny form adds elements of grey/silver to it, to tie in with the concept of the Phoenix being reborn from its own ashes (hence the grey colour). And its fire-type reflects the flames in which the Phoenix combusts as it dies and is reborn. While Metagross goes from being blue-ish to silver and gold, which are metals surely, but then again, why is it blue to start with? So Ho-Oh it is.
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VGC, Friend Safari, Masuda
VGC, Friend Safari, Masuda